If you ordered from EU, you didn't get a preorder code, just early access code. Seeing as your mogstation shows Heavenward early access, you don't need to do anything more yet.

If you preordered the physical version, then once it's delivered, you'll have to enter the CD key in the box into the mogstation, I'm assuming instructions will be in the box. This must be done before the 27th, as that's when the early access period ends.

For digital versions, I can't be certain, but I'm assuming it's the same, only the CD key will be emailed to you. Don't quote me on that though.