"The other app" ?
So you're not trying to have your application send to the clipboard...
You're trying to programmatically control someone else's application, and make it think a user has pressed control-c.
Does that sound like a correct clarification?
My first guess is that the *window* has nothing to send to the clipboard.
But maybe a control *in* the window does. Forms don't generally have any content to go to the clipboard, but a textbox for example has a .Text property that has text that can go on the clipboard.