How it works: go to https://store.galaxy-of-heroes.starwars.ea.com/redeem, sign in with your account, and type the code. The reward shows up in your in-game inbox.
Working today: MAUL, OLDBEN, DINDJARIN, REBEL
Heads up — each code is one use per account and they can expire without notice, so redeem them while they work. They've all been character or faction names in caps so far, so there are probably more out there. If you've got one that isn't on the list, drop it in the comments and I'll add it.