In a post, do you mean?
Well, you first click the little world button that has a link on it, and paste the website you want to link to in it, and click "OK". You'll then start out with a tag that looks like this:
[ URL=http://www.mywebpage.com]
http://www.mywebpage.com[/URL ]
The words that I've highlighted in red are the words that you can change to say whatever you want. By default this area is selected after you click ok, so normally you can just start typing and it will change the long website to the words you want.
So if you changed it to:
[ URL=http://www.mywebpage.com]
Go HERE[/URL ]
the post would just say "Go HERE" but clicking on those words would take you to the website that's listed in the left of the tag.
I hope that helps. :original: