Can you purchase a World of warcraft online game card for the free trail accounts?

I have a WOW free trail account and would like to upgrade my account and don't really know any other way to do it, and was wondering if the online game cards that they have would work on a free trail account as well. When I start the game it says that I would have to upgrade my account and this is a way to upgrade an account, I just want to know it would work on my free trail account before I buy

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