Note that I'm adding an extra space between the open bracket and the command so that it FA shows instead of parsing it.
Start with [ url]
End the link with [ /url]
(It's not case sensitive, BTW.)
You can turn text into a link, too. Start with [ url=http://www.some.url.or.other.com]
Add whatever text you want the link to be.
End with [ /url] as usual.
In many cases you don't really need to make a relative link, FA will detect the URL and make it a link automatically. Personally, I prefer to always make a relative link 'cause I'm just paranoid that way.

BTW, this page has more info that you might find useful, like how to make coloured text or © opyright symbols. -- http://www.furaffinity.net/journal/441426/

Thank you for the link, I had forgotten about that post. :)

You're welcome.

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