I'm not 100% sure what you're asking. Firstly, are you saying that you wrote that code yourself? If so, why? If you didn't do so already, you should have simply added ContextMenuStrip to the form in the designer and then added the desired items to it. If you wouldn't add TextBoxes and Buttons to the form using hand-coding then why do it for ContextMenuStrips?

Secondly, what do you mean by "inherit"? If you mean that you want to add a TextBox to that form and have it display that menu when it's right-clicked then all you need to do is select that menu as the value for the ContextMenuStrip property of the TextBox. If you can't do that already then that may be because the code you've written by hand is slightly different to what the designer would generate. If you let the designer generate the code then you should have no issue.

If you mean something other than that by "inherit" then you'll have to explain. "Inherit" usually means one class being derived from another but that's not possible with a TextBox and a Form and I can't imagine that that's what you would be talking about.