Redeem codes are entered by new players in the first 24 hours of playing and ONLY in the first 24 hours of playing. If you can enter a referral code, there is a line for it in the redeem tab (located in the "Messages" menu), under your own code. Here's an example of how referral codes work:
You send your referral code to a friend who has never played the game before.
Once your friend installs the game and completes the tutorial, they will be asked to input a referral code. This is where they will enter YOUR referral code.
You will be rewarded with a Redeem Point for each player who has entered your code upon completing the tutorial.
Once you have collected enough Redeem Points, you can exchange them for a reward at the Redeem Screen.
You send your referral code to a friend who has never played the game before.
Once your friend installs the game and completes the tutorial, they will be asked to input a referral code. This is where they will enter YOUR referral code.
You will be rewarded with a Redeem Point for each player who has entered your code upon completing the tutorial.
Once you have collected enough Redeem Points, you can exchange them for a reward at the Redeem Screen.