Also when you're on the reply to topic screen right above the message window one of the icons is a globe with a chain link on it, that's the url button. If you click on that a little message window will pop up in tere you write what you want the link to say and click ok, next another message windo comes up where you put the site address in, click ok again and you're done.
This puts it at the end of anything that's already been typed so keep that in mind when using it.