Make a Form a child of an existing running app?

[Dan Baker]

I have a already-running application, that has a menu-option that I want to
launch a C# app to process.

The C# app shows a form, and when closed, goes back to the parent app. I
have done this in the past by passing the parent window handle (hwnd) to the
child application on the command line, and then using that handle as the
"handle to parent" when the dialog was created.

How do I create a form given an hwnd to an existing window?

Thanks
DanB