For those who have pre-purchased Stormblood or Heavensward, please refer to these activation instructions as the instructions below are for the base game.
Follow one of the links below, depending on your Region.
Create to your SE account and access the Mogstation. When you have logged in, press "Add new service account"
Enter your code as shown below
A confirmation page will appear detailing your actions
Once you press confirm and accept the Terms and conditions on the next page, your key will be registered on your account.
You will be required to setup a recurring monthly subscription to utilise any free trials that may have come with the game.