You can't. They already disabled it to prevent further botters. Your going to have to wait until they release the new RaF program, which from my understanding will transfer over:

>With the new update, for every friend you refer who reaches level 10, you’ll earn 1000 IP. 

>There are no additional rewards to earn, and everyone who has participated in Refer-a-Friend in the **past** will be transferred to the new program with the same number of referrals as they currently have. For example, if you’ve already referred four friends, you’ll earn 4000 IP and Grey Warwick. 

So you'll still get the intended rewards if you have referred someone.

[Source 1][1]


  [1]: http://na.leagueoflegends.com/en/news/game-updates/features/upcoming-changes-refer-friend