When you post a new message there will be a little globe with a paper clip on it in the controls at the top of the new message screen. You click on that to enter a link. A good thing to do is type in some words, which you can make bold, or underline, or change the font, then highlight those words, and then click on the link tool and enter the link. For instance, I can enter a link like this: Cruise Reviews Forums - Powered by vBulletin
or I can do it this way: CruiseReviews forum.