I had an issue where the codes for pre-order in my email were not working, and were giving a "invalid code" message, so I popped along to support and they told me how to fix it (thanks Victoria!).

Problem is, the code in the email is different to the code required, so it doesn't work.

Solution
  • Go to the SE Store
  • Log in (naturally)
  • Click "My Profile" under My Account

To the right you will see a Red button, it says View your FFXIV Heavensward Early Access

Click that and you'll get your pre-order code, that one should then work on the pre-order page in Mogstation. I'm in the EU, so no idea if it works for NA players, and of course, only works if you bought it from the SE store.