First, as you edit your forum post, be sure that 'ezCodes' is checked on just above the edit window. This turns a bunch of formatting controls on.

Then click the link button and a window will appear for you to include an URL to link to. Click OK twice and it will insert a bunch of code that inserts a link:

by default, the link text is the URL you pasted in, but you can change this to anything else :

visit the airheads-chat rogues' gallery page!

While you're looking at links, notice that every forum page has an URL you can copy and paste into email so people can jump directly to that forum post. I recently posted one like this to the airheads-chat list, as you may recall.

So you could link to this page by using the URL