How to perform mail merge in ASP.NET MVC?

<div>
            <div style="margin-bottom:12px">
                <button onclick="mergeDocument()" class="e-control e-btn e-primary">Merge Document</button>
            </div>
            <div style="display:flex">
                <div style="width:15%;border:1px solid #e0e0e0">
                    <b><label style="display:block;margin:10px">Select Field to Insert</label></b>
                    @Html.EJS().ListView("listview").Enable(true).Select("onFieldSelect").DataSource((IEnumerable<object>)ViewBag.dataSource).Render()
                </div>
                <div style="width:85%;border:1px solid #e0e0e0">
                    @Html.EJS().DocumentEditor("container").IsReadOnly(false).EnableSelection(true).EnableEditor(true).EnableEditorHistory(true).EnableWordExport(true).Created("onCreated").Render()
                </div>
            </div>
            <div class="overlay" id="popup-overlay" style="display: none;"></div>
            <div id='waiting-popup' style="display: none;">
                <svg class="circular" height="40" width="40">
                    <circle class="circle-path" cx="25" cy="25" r="20" fill="none" stroke-width="6" stroke-miterlimit="10" />
                </svg>
            </div>
        </div>

 
    var documenteditor;
    var listView;
 
    function onCreated() {
        documenteditor = document.getElementById("container").ej2_instances[0];
        JS documenteditor.pageOutline ="#e0e0e0"
        listView = document.getElementById("listview").ej2_instances[0];
        documenteditor.resize();
    }
 
    function onFieldSelect(args) {
        let fieldName = args.data.text;
        listView.selectItem(undefined)
        insertField(fieldName);
    }
 
    function insertField(fieldName) {
        fileName = fieldName.replace(/\n/g, '').replace(/\r/g, '').replace(/\r\n/g, '');
        var fieldCode = 'MERGEFIELD  ' + fileName + "  \\* MERGEFORMAT ";
        documenteditor.editor.insertField(fieldCode, '«' + fieldName + '»');
        documenteditor.focusIn();
    }
 
    function mergeDocument() {
        documenteditor.saveAsBlob("Docx").then(function (blob) {
            var fileReader = new FileReader();
            fileReader.onload = function () {
                var base64String = fileReader.result;
                var data = {
                    fileName: documenteditor.documentName + '.docx',
                    documentData: base64String
                }
                showHideWaitingIndicator(true)
                var httpRequest = new XMLHttpRequest();
                httpRequest.open('Post', '/api/DocumentEditor/MailMerge', true);
                httpRequest.setRequestHeader("Content-Type", "application/json;charset=UTF-8");
                httpRequest.onreadystatechange = function () {
                    if (httpRequest.readyState === 4) {
                        if (httpRequest.status === 200 || httpRequest.status === 304) {
                            documenteditor.open(httpRequest.responseText);
                        } else {
                            // Failed to merge document
                        }
                        showHideWaitingIndicator(false);
                    }
                };
                httpRequest.send(JSON.stringify(data));
            }
            fileReader.readAsDataURL(blob);
        });
    }
 
    function showHideWaitingIndicator(show) {
        document.getElementById("popup-overlay").style.display = show ? 'block' : 'none';
        document.getElementById("waiting-popup").style.display = show ? 'block' : 'none';
    }

public ActionResult Index()
        {
            List<object> listdata = new List<object>();
            listdata.Add(new
            {
                text = "FirstName",
                id = "_firstname",
            });
            listdata.Add(new
            {
                text = "LastName",
                id = "_lastname",
            });
            listdata.Add(new
            {
                text = "CustomerID",
                id = "_customerid",
            });
            listdata.Add(new
            {
                text = "EmailID",
                id = "_emailid",
            });
            listdata.Add(new
            {
                text = "MobileNo",
                id = "_mobileno",
            });
            ViewBag.dataSource = listdata;
            return View();
        }

[HttpPost]
        public HttpResponseMessage MailMerge([FromBody]ExportData exportData)
        {
            Byte[] data = Convert.FromBase64String(exportData.documentData.Split(',')[1]);
            MemoryStream stream = new MemoryStream();
            stream.Write(data, 0, data.Length);
            stream.Position = 0;
            try
            {
                Syncfusion.DocIO.DLS.WordDocument document = new Syncfusion.DocIO.DLS.WordDocument(stream, Syncfusion.DocIO.FormatType.Docx);
 
                document.MailMerge.RemoveEmptyGroup = true;
                document.MailMerge.RemoveEmptyParagraphs = true;
                document.MailMerge.ClearFields = true;
 
                document.MailMerge.Execute(CustomerDataModel.GetAllRecords());
                SplitEachRecords(document);
 
                document.Save(stream, Syncfusion.DocIO.FormatType.Docx);
            }
            catch (Exception ex)
            {
 
            }
            // Write Pdf stream here.
            return new HttpResponseMessage() { Content = new StringContent(ImportWordDocument(stream)) };
 
        }
 
        public string ImportWordDocument(Stream stream)
        {
            string sfdtText = "";
            Syncfusion.EJ2.DocumentEditor.WordDocument document = Syncfusion.EJ2.DocumentEditor.WordDocument.Load(stream, Syncfusion.EJ2.DocumentEditor.FormatType.Docx);
            sfdtText = Newtonsoft.Json.JsonConvert.SerializeObject(document);
            document.Dispose();
            return sfdtText;
        }
        private void SplitEachRecords(Syncfusion.DocIO.DLS.WordDocument document)
        {
            TextSelection[] textSelections = document.FindAll("#PageBreak", true, true);
 
            if (textSelections != null)
            {
                foreach (TextSelection selection in textSelections)
                {
                    WParagraph paragraph = selection.GetAsOneRange().OwnerParagraph;
                    paragraph.Text = string.Empty;
                    paragraph.InsertSectionBreak(SectionBreakCode.NewPage);
                }
            }
        }
        private void ApplyBoldFormatting(object sender, MergeFieldEventArgs args)
        {
            if (args.FieldName == "IsDefault")
            {
                if (args.FieldValue.ToString() == "Yes")
                    isApplyBold = true;
                else
                    isApplyBold = false;
            }
            else if (args.FieldName == "FirstName")
            {
                if (isInsertBreak)
                {
                    bool isInCell = args.CurrentMergeField.OwnerParagraph.Owner is WTableCell;
                    bool isInTextBox = args.CurrentMergeField.OwnerParagraph.Owner is WTextBox;
                    bool isInContentControl = args.CurrentMergeField.OwnerParagraph.Owner is BlockContentControl;
 
                    WParagraph paragraph;
                    if (isInCell)
                    {
                        WTableCell ownerCell = args.CurrentMergeField.OwnerParagraph.Owner as WTableCell;
                        paragraph = ownerCell != null ? (ownerCell.OwnerRow.Owner as WTable).PreviousSibling.PreviousSibling as WParagraph : args.CurrentMergeField.OwnerParagraph.PreviousSibling as WParagraph;
                    }
                    else if (isInTextBox)
                    {
                        WTextBox textBox = args.CurrentMergeField.OwnerParagraph.Owner as WTextBox;
                        paragraph = textBox.OwnerParagraph.PreviousSibling.PreviousSibling is WParagraph ? textBox.OwnerParagraph.PreviousSibling.PreviousSibling as WParagraph : args.CurrentMergeField.OwnerParagraph;
                    }
                    else if (isInContentControl)
                    {
                        BlockContentControl contentControl = args.CurrentMergeField.OwnerParagraph.Owner as BlockContentControl;
                        paragraph = contentControl.PreviousSibling.PreviousSibling as WParagraph;
                    }
                    else
                        paragraph = args.CurrentMergeField.OwnerParagraph.PreviousSibling.PreviousSibling as WParagraph;
 
                    paragraph.Text = "#PageBreak";
                }
                else
                    isInsertBreak = true;
            }
 
            if (isApplyBold)
            {
                foreach (Entity entity in args.CurrentMergeField.OwnerParagraph.Items)
                {
                    if (entity is WTextRange)
                        (entity as WTextRange).CharacterFormat.Bold = true;
                }
                // Apply the bold formatting to the adjacent cell.
                WTableCell cell = args.CurrentMergeField.OwnerParagraph.Owner.PreviousSibling as WTableCell;
                ApplyBoldFormattingForCell(cell);
            }
        }
        private void ApplyBoldFormattingForCell(WTableCell cell)
        {
            foreach (WParagraph paragraph in cell.Paragraphs)
            {
                foreach (Entity entity in paragraph.Items)
                {
                    if (entity is WTextRange)
                        (entity as WTextRange).CharacterFormat.Bold = true;
                }
            }
        }