How to add new tab at specific index poistion for WPF DockingManager?

//How to Add  new tab at specific index position for dockingmanager
<Window x:Class="DocumentTabOrder.MainWindow"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:syncfusion="http://schemas.syncfusion.com/wpf"
                 Title="DocumentTabOrderIndex" Height="350" Width="525">  
      <Grid>
           <Grid.RowDefinitions>
                <RowDefinition Height="40" />
                <RowDefinition/>
           </Grid.RowDefinitions>
           <Button Content="add tab" Height="30" Width="200" VerticalAlignment="Center" HorizontalAlignment="Center" Click="AddItem"/>
           <syncfusion:DockingManager Grid.Row="1" x:Name="dockingManager" UseDocumentContainer="True">
               <ContentControl syncfusion:DockingManager.State="Document" syncfusion:DockingManager.Header="Tab1"  syncfusion:DockingManager.DocumentTabOrderIndex="1">
               </ContentControl>
               <ContentControl syncfusion:DockingManager.State="Document" syncfusion:DockingManager.Header="Tab2" syncfusion:DockingManager.DocumentTabOrderIndex="2">
               </ContentControl>
               <ContentControl syncfusion:DockingManager.State="Document" syncfusion:DockingManager.Header="Tab3" syncfusion:DockingManager.DocumentTabOrderIndex="3">
               </ContentControl>
               <ContentControl syncfusion:DockingManager.State="Document" syncfusion:DockingManager.Header="Tab4" syncfusion:DockingManager.DocumentTabOrderIndex="4">
               </ContentControl>
               <ContentControl syncfusion:DockingManager.State="Document" syncfusion:DockingManager.Header="Tab5" syncfusion:DockingManager.DocumentTabOrderIndex="5">
               </ContentControl>
           </syncfusion:DockingManager>
      </Grid>
</Window>

//How to Add  new tab at specific index position for dockingmanager
namespace DocumentTabOrder
{
  public partial class MainWindow : Window
  {
    public MainWindow()
    {
      InitializeComponent();
    }
    private void AddItem(object sender, RoutedEventArgs e)
    {
      Grid grid1 = new Grid();
      DockingManager.SetDocumentTabOrderIndex(grid1, dockingManager.Children.Count);
      DockingManager.SetHeader(grid1, "Tab " + (dockingManager.Children.Count + 1));
      DockingManager.SetState(grid1, DockState.Document);
      dockingManager.Children.Add(grid1);
    } 
  }
}