The form needed by this forum is very strictly (without spaces):
http : // youtube.com / watch ? v=ABCDEFGH
where ABCDEFGH is the video identifier.
When you've got a youtube video open in a browser, the form in the address bar will usually be close enough to what you need, except it will start with https (kill the s), and will probably have some junk after the video identifier starting with "&" (get rid of all of it, including the "&"). Bang what you've got left between [ youtube ] tags, and you should be right to go.
The forum software could do all that for you, but it's ancient and also shit, so it doesn't.