Hi Kim Johansen,
Thank you for the question.
RadTextBox hosts the Microsoft TextBox control for its textbox part. When you set the ContextMenuStrip property of RadTextBox, the contextmenu is shown when you right-click on the RadElements of the RadTextBox (which are available close to the border). In order to get the custom contextmenu in the textbox part as well, please set it explicitly to the hosted TextBox:
You can also use our RadContextMenu in RadTextBox. Please refer to the following help article for additional information: Assign RadContextMenu to Telerik and non-Telerik controls
If you have additional questions, feel free to contact me.
the Telerik team