public event PopupMenuShowingEventHandler PopupMenuShowing

Public Event PopupMenuShowing As PopupMenuShowingEventHandler

<dxre:RichEditControl Name="richTextEditor" 
                      PopupMenuShowing="richTextEditor_PopupMenuShowing">

using DevExpress.XtraRichEdit.API.Native;
using DevExpress.Xpf.RichEdit;
using DevExpress.Xpf.Bars;
using DevExpress.Xpf.RichEdit.Menu;
using DevExpress.XtraRichEdit.Commands;
// ...

private void richTextEditor_PopupMenuShowing(object sender, PopupMenuShowingEventArgs e) {
    // Check whether the event is raised for document text.
    if (e.MenuType == RichEditMenuType.Text) {
        // Create a menu item to insert a picture
        // and bind this item to the Rich Text Editor's command.
        e.Customizations.Add(new BarButtonItem() {
            Command = RichEditUICommand.InsertFloatingPicture,
            Content = "Insert Picture",
            CommandParameter = richTextEditor
        });

        // Create a custom menu item to highlight selected text.
        var menuItem = new BarButtonItem() {
            Name = "highlightSelectionItem",
            Content = "Highlight Selection"
        };
        menuItem.ItemClick += MenuItem_ItemClick; ;
        e.Customizations.Add(menuItem);
    }

    // Remove the "Increase Indent" item from the menu.
    e.Customizations.Add(new RemoveRichEditCommandAction() {
        Id = RichEditCommandId.IncreaseIndent
    });

    // Remove the "Decrease Indent" item from the menu.
    e.Customizations.Add(new RemoveRichEditCommandAction() {
        Id = RichEditCommandId.DecreaseIndent
    });
}

private void MenuItem_ItemClick(object sender, ItemClickEventArgs e) {
    var selectedRanges = richTextEditor.Document.Selections;
    foreach (var range in selectedRanges) {
        var charProps = richTextEditor.Document.BeginUpdateCharacters(range);
        charProps.BackColor = System.Drawing.Color.Yellow;
        richTextEditor.Document.EndUpdateCharacters(charProps);
    }
}

Imports DevExpress.XtraRichEdit.API.Native
Imports DevExpress.Xpf.RichEdit
Imports DevExpress.Xpf.Bars
Imports DevExpress.Xpf.RichEdit.Menu
Imports DevExpress.XtraRichEdit.Commands
' ...

Private Sub richTextEditor_PopupMenuShowing(ByVal sender As Object, ByVal e As PopupMenuShowingEventArgs)
    ' Check whether the event is raised for document text.
    If e.MenuType = RichEditMenuType.Text Then
        ' Create a menu item to insert a picture
        ' and bind this item to the Rich Text Editor's command.
        e.Customizations.Add(New BarButtonItem() With {
            .Command = RichEditUICommand.InsertFloatingPicture,
            .Content = "Insert Picture",
            .CommandParameter = richTextEditor
        })

        ' Create a custom menu item to highlight selected text.
        Dim menuItem As New BarButtonItem() With {
            .Name = "highlightSelectionItem",
            .Content = "Highlight Selection"
        }
        menuItem.ItemClick += MenuItem_ItemClick
        e.Customizations.Add(menuItem)
    End If

    ' Remove the "Increase Indent" item from the menu.
    e.Customizations.Add(New RemoveRichEditCommandAction() With {
        .Id = RichEditCommandId.IncreaseIndent})

    ' Remove the "Decrease Indent" item from the menu.
    e.Customizations.Add(New RemoveRichEditCommandAction() With {
        .Id = RichEditCommandId.DecreaseIndent})
End Sub

Private Sub MenuItem_ItemClick(ByVal sender As Object, ByVal e As ItemClickEventArgs)
    Dim selectedRanges As SelectionCollection = richTextEditor.Document.Selections
    For Each range As DocumentRange In selectedRanges
        Dim charProps As CharacterProperties = richTextEditor.Document.BeginUpdateCharacters(range)
        charProps.BackColor = System.Drawing.Color.Yellow
        richTextEditor.Document.EndUpdateCharacters(charProps)
    Next range
End Sub

<dxre:RichEditControl x:Name="richEdit" CommandBarStyle="Empty" BarManager="{Binding ElementName=barManager1, Mode=OneTime}" PopupMenuShowing="RichEditPopupMenuShowing">
    <dxre:RichEditControl.MenuCustomizations>

#line 38 "..\..\..\MainWindow.xaml"
this.richEdit.PopupMenuShowing += new DevExpress.Xpf.RichEdit.PopupMenuShowingEventHandler(this.RichEditPopupMenuShowing);

#ExternalSource("..\..\MainWindow.xaml",38)
AddHandler Me.richEdit.PopupMenuShowing, New DevExpress.Xpf.RichEdit.PopupMenuShowingEventHandler(AddressOf Me.RichEditPopupMenuShowing)