As you see here:
https://account.swtor.com/user/account
The account activation process is in place, so if you have a retail key already and want to set up a sub, there you go.
NOTE: This is for the Retail key, which will be needed past the 20th of December, It is not for the pre-order key as you redeemed that already anyway!
Edit: For those scared to click links, swtor.com then click account and log in.