Okay lets say you bought this, once the whole purchase is complete and you get a confirmation email.
you want to find the product key by clicking on the "cd key" link to the right side of steam while you have wakfu selected
in the list you want to select the product key of what you purchased and copy to clipboard
*IMPORTANT* make sure you're logged into the account you want to receive the package on before accepting the gift code
from there, you want to proceed to the wakfu website and go to the bottom of the page where you'll find "gift codes" under customer service.
Log in to the desired account and paste the code in and hit confirm.
and that's it, enjoy your new stuffs.