>A VFP Application has to send a string to a non-VFP Application, i try it now with WM_KEYDOWN / WM_KEYUP...

If you're going to send it as a string as keystrokes, rather than sending keystroke by keystroke, why not just fill the clipboard with the string and send a Ctrl-V to get the other app to paste the content of the clipboard? Either that, or look into using the Wscript.Shell's AppActivate and SendKeys methods to do this.

You definitely want to use PostMessage() rather than SendMessage() here since you don't need to wait for responses to each individual message if you use windows messages. I think it'd be a tremendous mistake to take this approach unless you have a way of guarenteeing the focus of the other app on the proper control to deal with the content, but that's just my opinion.