1) Log into the SQUARE ENIX Account Management System (
https://secure.square-enix.com) using the SQUARE ENIX account that is tied to the service account (PlayOnline ID) you are playing on.
2) Select “PlayOnline / FINAL FANTASY XI.”
3) Click on the “Add a service account” button and enter your campaign or promotional code.
4) Follow the onscreen instructions and select the intended service account (PlayOnline ID).
5) Carefully review the information displayed in the confirmation window and complete the registration.