You can add codes to your Blizzard account through the website or through the Battle.net App.
Claim Your Code on the Website
To claim a code on the website:
Log into your Account Overview https://account.blizzard.com/overview
Enter the code in the Redeem a Code box and click Redeem Code
Claim Your Code on the App
To claim a code on the Battle.net App:
From the Games tab, click the Redeem a Code button found under the logo of any game
From the Shop tab, click Services and then Redeem a Code