You can use a coupon in either of the following two ways:
① In-Game
Go to Settings → Account → Enter Code
Enter your coupon code, and the reward will be delivered to your in-game mailbox.
② Web
Access the coupon redemption page via the link below, then enter your UID, Server, and Code.
👉 Coupon Page: http://ec2-54-202-180-51.us-west-2.compute.amazonaws.com:39009/coupon