You can also use the BBCode built into the forum if that's where your wanting to shorten urls.

For example you have this http://www.superlongneverendingurlth...oonforever.com

Code:
url=http://www.superlongneverendingurlthatwillgoonforever.com]Linked Text[/url]
Will display this in your actual message:

Linked Text

Note the example in the "code" box has the first bracket [ removed otherwise it would just have linked text in that box also so you need that first bracket [