When replying to a forum topic I see this text if I click to open the 'Input forum' dialog. (I also see something similar when starting a new topic, but it isn't titled.)
Link to content with ((some text)), where "some text" is the title of existing content or the title of a new piece of content to create. You can also link text to a different title by using ((link to this title|show this text)). Link to outside URLs with ((http://www.example.com|some text)), or even ((http://www.example.com)).
PRE-POST EDIT: I have had to swap out the square brackets for different ones, as the forum software was changing them.
However, trying to link to an external site throws me over to the freelinking do-dad, which tries to create a wiki page instead.