Android and Referer

How do I have a friend use me as a refer on the android?

I have had him try on different devices now. I don't see an option in My Account. And when they start the game they didn't have any luck with a refer option either.

Please help. I'd really like him to be on the same server I am.

thanks

have him complete task 4 then task 5 should appear( it asks for refferal there)

Ty for responding back. By that time then they are already on a different server? I thought the whole refer thing also put the friends on the same server relatively close to each other.