Just joined but don't know how to credit the friend that referred me to bulb?

That’s the question! I just joined bulb but in the sign up process there was no where to acknowledge the friend that referred me.
I gather now that I should have asked him to send me a link and used that to sign up but I didn’t know that at the time.

So how do I credit the friend that referred me to bulb now I’ve signed up?

Thanks.