public event ShowingMenuEventHandler ShowingMenu

Public Event ShowingMenu As ShowingMenuEventHandler

private void dockManager1_ShowingMenu(object sender, DevExpress.Xpf.Docking.Base.ShowingMenuEventArgs e) {
    ItemContextMenu menu = (e.Menu as ItemContextMenu);
    if (menu == null) return;
    // Action 1 - Insert a new About button at the first position in the menu.
    BarButtonItem actionAddAboutItem = new BarButtonItem();
    // Specify the position for the button via the InsertBarItemLinkAction.ItemLinkIndex attached property.
    InsertBarItemLinkAction.SetItemLinkIndex(actionAddAboutItem, 0);
    actionAddAboutItem.Content = "About...";
    actionAddAboutItem.ItemClick += new ItemClickEventHandler(btnAbout_ItemClick);
    // Action 2 - Insert a separator
    InsertBarItemLinkAction actionAddSeparator = new InsertBarItemLinkAction();
    actionAddSeparator.ItemLink = new BarItemLinkSeparator();
    actionAddSeparator.ItemLinkIndex = 1;

    // Add the actions to the ActionList
    e.ActionList.Add(actionAddAboutItem);
    e.ActionList.Add(actionAddSeparator);
}

void btnAbout_ItemClick(object sender, ItemClickEventArgs e) {
    MessageBox.Show("About window");
}

Private Sub dockManager1_ShowingMenu(ByVal sender As Object, ByVal e As DevExpress.Xpf.Docking.Base.ShowingMenuEventArgs)
    Dim menu As ItemContextMenu = (TryCast(e.Menu, ItemContextMenu))
    If menu Is Nothing Then
        Return
    End If
    ' Action 1 - Insert a new About button at the first position in the menu.
    Dim actionAddAboutItem As New BarButtonItem()
    ' Specify the position for the button via the InsertBarItemLinkAction.ItemLinkIndex attached property.
    InsertBarItemLinkAction.SetItemLinkIndex(actionAddAboutItem, 0)
    actionAddAboutItem.Content = "About..."
    AddHandler actionAddAboutItem.ItemClick, AddressOf btnAbout_ItemClick
    ' Action 2 - Insert a separator
    Dim actionAddSeparator As New InsertBarItemLinkAction()
    actionAddSeparator.ItemLink = New BarItemLinkSeparator()
    actionAddSeparator.ItemLinkIndex = 1

    ' Add the actions to the ActionList
    e.ActionList.Add(actionAddAboutItem)
    e.ActionList.Add(actionAddSeparator)
End Sub

Private Sub btnAbout_ItemClick(ByVal sender As Object, ByVal e As ItemClickEventArgs)
    MessageBox.Show("About window")
End Sub

Name="dockManager1"
ShowingMenu="dockManager1_ShowingMenu">
<dxdo:LayoutGroup x:Name="rootGroup" Orientation="Horizontal">

#line 12 "..\..\..\Window1.xaml"
this.dockManager1.ShowingMenu += new DevExpress.Xpf.Docking.Base.ShowingMenuEventHandler(this.dockManager1_ShowingMenu);

#ExternalSource("..\..\Window1.xaml",12)
AddHandler Me.dockManager1.ShowingMenu, New DevExpress.Xpf.Docking.Base.ShowingMenuEventHandler(AddressOf Me.dockManager1_ShowingMenu)