How to override defualt textbox context menu in windows store c#

doesnt look like you can extend the contextmenu but you can override it, Use the ContextMenuOpening event

private void TextBox_ContextMenuOpening(object sender, ContextMenuEventArgs e)
        {
            e.Handled = true;

            Popup f = new Popup();
            f.IsLightDismissEnabled = true;
            f.Child = new Border
                {
                    Background = new SolidColorBrush(Colors.CadetBlue),
                    Width = 200,
                    Height = 100
                };

            f.VerticalOffset = e.CursorTop;
            f.HorizontalOffset = e.CursorLeft;

            f.IsOpen = true;


          
        }

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Microsoft Certified Solutions Developer - Windows Store Apps Using C#