If you are using Steam, open your internet browser and sign in to www.thehunter.com using your email and password. Otherwise you can use the non-Steam launcher.
Click on 'Buy em$' or 'Upgrade / Renew', then 'Redeem a discount or gift code'
The gift code should have the following format. Copy and paste it in the provided box.
Please note that it's different from Retail Activation Code. If your gift code is valid, you should see the contents of it, It's only one click away from being credited to your account.