Below you can find a list of the coupons that are active as of now

GT4YEAR - available on EU, NA, LA, OC, and Asia servers. 

EPICFOURTH - available on EU, NA, LA, and OC servers. Asia will receive the reward of this one differently.

Q. How to claim codes?
A. For Android, either in-game or using this site. 
For iOS, only using this site.

Q. Are they available on all servers and platforms? 
A. Please note that Kakao Games handles the global version (EU/NA/LA/OC), and most of the codes will be available on our servers for mobile. There can be exceptions where SEA can redeem codes we share, like the one(s) from the stream(s).
These codes are not redeemable on Nintendo Switch.