What sort of code have you got?

If it's 18 characters long (in the format 6-3-6-3) then you go to https://redeem.finalfantasyxiv.com/ to redeem it. Although they haven't yet started sending them out, once they do they'll send you a 20 character code (4-4-4-4-4) which you put into the mogstation like normal.

If you already have a 20 character code then just hang on to it for now, it'll be redeemable at somepoint in the future (probably when they start sending codes to the 18 code people)