I don't use flash so this is just a guess but it seems likely you'd have to do it in the flash code.
What I really want to say is that if you MUST use a flash intro, put a link on the intro that allows the user to skip it. Many people find flash intros to be highly irritating. I, for example, am one of those people who will leave a site immediately if the flash intro doesn't have a "skip" link.
Better yet, drop the flash intro. 🙂