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you can't, you have to choose all of the game at once.

I think it is a product code or smth, which will be associated with your account when you make your purchase with AMD. If you are planning to get from other 3rd party sources then you need to use it immediately, (In which you have to select what games you want at that point).

"after i buy it can i only register it to my amd account and not choose any games when i register the key?" - wtf?!

You enter the voucher code on AMD page and then you can pick 3 games. If you want to wait another month, just wait another month with redeeming the code...