In general, if you click POST REPLY (or if you click Quick Reply and then change your mind by clicking Full Editor) you can always get to the "help screen" for composing by clicking the
BBCode link on the far right, under the smilies.
If you're familiar with HTML, BBCode will be
similar, but the actual tags and markup are slightly different. You can usually get away with highlighting a few words and then clicking the formatting buttons, but in the case of the URL button, it seems to leave out the most important part of the markup!