One of the codes has already been used, if the one you tried doesn't work try with another one.

Recruitment Code 1: 9WFZSZ7D
Recruitment Code 2: 83C36GZV
Recruitment Code 3: NS3YUAYB
Recruitment Code 4: 9XFYNC3A

Remember that you need to use the code before getting a paid subscription — that is, during your free month after purchasing the game. I play on Cerberus, so if you need help with anything send me a /tell or a mail ingame (or even message me on the forums), my IGN is Zen Shiwaburo.

To use the code:

1. After creating a Square Enix account, log into the Mogstation.
2. Go to "Your Account", then find the "Enter Recruitment Code" button. Click on it and enter the code.
3. Enjoy! Your rewards will be given to you once you get a paid subscription, but if you want them as soon as possible you can get a game-time card which gives you 60 days of game time. The rewards are the following:

Friendship Circlet: Head equipment. Increases EXP earned by 20% when level 25 and below.
Aetheryte Ticket x99: Using Teleport will consume one of these tickets instead of gil.
Ballroom Etiquette - Improper Greetings: This item will unlock a fistbump emote.
10 Silver Chocobo Feathers: You can trade these for good low level gear or even level 50 gear.