Per Kaden on FFXIVrealm
Currently even though the page is live to the public, it's not actually working as intended.
Don't use the
http://preorder.eu.finalfantasyxiv.com/ (or na / jp) link(s) as it won't work.
Everyone will be notified, that entered their email address into the official site previously as to when the page is actually 'actively functioning' to redeem your codes.
The XXXXX-XXX-XXXXXX-XXX , 18 digit code you receive from your retailer you purchased the pre-order from IS the code you use @
http://preorder.eu.finalfantasyxiv.com/ but only to be used when the community (as a whole) is notified via email that the page is up and functioning officially for use. Support advised me to advise anyone, not to use the page until said notification email is received (he approximated within the next 24/48 hours)
I asked them "Is my code now void..." because the game is now registered against my SE account but on PC instead of PS3, they reassured me that the code will not be void and to try it again when the page is active. If it doesn't work, then to phone them directly and they will address it.