How to change what appears when you right-click in a rich text box in C #?

     ContextMenu contextMenu = 
        new System.Windows.Forms.ContextMenu (
           new [] {
             new MenuItem ("Entry1", ContextMenuItem_Click),
             new MenuItem ("Copy", ContextMenuItem_Click)
           });
     RichTextBox rtb = 
       new RichTextBox
         {
            Size = ClientSize,
            Parent = this,
            ContextMenu = contextMenu
         };