How to add my digital game time to my world of warcraft trial account?

hey guys i just bought 30 days game time off the blizzard store and i don’t know how to add it so i have it activated, nothing seems to be working and its telling me

You have entered a World of Warcraft® key that is currently not supported in Battle.net account management. This type of key can be redeemed through the World of Warcraft® account management page.

what do i do?