You need to own a copy of the game in order for a prepaid subscription card to work.
I have used prepaid for my sub since release.
Edited by Samadhi on July 22, 2014 8:56PM
"If you want others to be happy, practice compassion. If you want to be happy, practice compassion." -- the 14th Dalai Lama Wisdom is doing Now that which benefits you later.
A. A copy of the game and a payment method for a recurring subscription. You will get a temporary authorization but will not be fully billed for the sub until your free month is up. This is the cheapest option because you can cancel sometime during the free month and not pay for additional time.
OR
B. A copy of the game and a game time card. This will allow you to get your free month and play without using a recurring sub, consequently allowing you to play without having a temporary authorization on any card. The downside is that you have to part with your money for the paid game time before you get your free month.
No matter what you do, you need to purchase the game itself first. You can buy it through the account page or you can purchase a copy from major retailers. It is currently $40 on Amazon. I'm sure you can find it even cheaper.