Thank you for writing.
You have to do two things in order to show a RadContextMenu in RadTextBox. First, you have to replace the default context menu with an empty one and then you have to show the RadContextMenu from the MouseDown event handler of the hosted control in the RadTextBox. I have attached a sample project where I have implemented these steps.
I hope this will help you. If you have further questions, do not hesitate to ask.
the Telerik team
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