It could be FindWindow. Are you using the actual Window's name or just "apptitle" ? And how is FindWindow declared in the .decls file ? For using it with a zero as a classname it should be:
apiFindWindow% (lpClassName%, lpWindowName$) : "FindWindowA"
(with a % instead of a $)
I haven't used it, but if you have the user32.decls, you could give "api_CloseWindow" a try.
apiFindWindow% (lpClassName%, lpWindowName$) : "FindWindowA"
(with a % instead of a $)
I haven't used it, but if you have the user32.decls, you could give "api_CloseWindow" a try.