Confused about registering account

I've been reading the forums about some people registering to wrong service account/starting new one accidently and since the whole thing confuses me could someone clear something. I want to know, when i get my game in few days(collectors edition for pc, physical) how do i register it the way that i get the items + 30 days free gametime to the service account i use?
I was playing in beta phase 4 so mog station has "FFXIV: A Realm Reborn - Beta Test Windows(R) is registered to this service account." and it's inactive(well, beta ended so..).Then under it there's also option to add plaftform, and it takes to page that wants me to enter the 20 digit number that i will get with the game, however the beginners quide(the one with moogle and catcirl) in Realm reborn site says that you can register the game in the game window.
So in which one i need to register mine to get the items etc to this service account instead of accidently making new one?

I don't use reddit too often, but this link ought to help, has a pic as well.
http://www.reddit.com/r/ffxiv/commen..._code_to_your/