is it a YOUTUBE video?
If you post a thread or post a response 'go advanced' you can use the shortcut button
to bring up the code or you can type it in
the full link is going to be something like:
http://www.youtube.com/watch?v=iRyb-RyQ1Ko
the part that you want to put in the brackets is only the part of the link after the equals sign =
So it would look like (without the spaces)
[ YOUTUBE ] iRyb-RyQ1Ko [ / YOUTUBE ]
which will make this video show:
....
I hope that helps.