Okay, so same problem. You can't create a post or thread purely for promoting the referral code... but if, say... the post tells about the event, the goals, and your friend... and then at the end of it you mention... "Oh and btw, here's his referral code."
That will be within forum rules. You just can't go promoting referral codes as the main purpose. The point is to regulate the number of nonsensical "Click this" posts.
So, so long as you word it well, it should be rather easy to do.