3 ways :
make a new steam account , activate the product on the new steam account , right click on the game in the steam list , select view product key and then copy to clipboard. now you have your new key , if this fails you will need to either ask someone who you trust who has mb to activate the product or contact steam as i am sure they will supply you with a key and invalidate the product redemption code as payment.
or generate a new key using the built in key checking within the warband error check it holds the next real value within eax. eventually it will give you a usable online key if you either breakpoint it and manualy record the next value or hook it that is upto you, it can take quite a while however to generate a working online key as only certain keys can be used.