Open this link: https://cdkey.lilith.com/afk-global, enter your in-game UID in the UID textbox and then open the game and tap on your user profile in the top left corner.

Go to the "settings" tab and then tap on "verification code", go back to https://cdkey.lilith.com/afk-global and enter the verification code number from the app. (Your verification code changes every 2 minutes).

Tap the log in button and enter the redemption code into the "gift code" box and tap the "Redeem" button. Your reward will be sent to the corresponding account's in-game mailbox.