The quick method is to click on the little Earth icon above, then paste in the URL....make sure to not have double "http://", because it automatically puts it in for you.

The code for doing it here is a little different than I'm used to in UBB boards....now the URL has to have double quotes around it.

[url="your_link_here"]custom_title_of_link[/ url] and take out the space I added.