I don't remember the exact template that is used, but I think I remember it being used in other places. It's not specific to registration. Putting it in that phrase is probably your best bet.
Do you use Google Analytics? If so what I did was set
http://yourdomain.com/register.php?do=addmember as a conversion goal. It keeps track of everything then. How many come from what ads as well as where all the other members who join come from. Very useful.