Hoorayy! You just received your Perk Gift Card email in your inbox, and you've clicked on "CLAIM MY PERK REWARD" and the next page is asking you for a Secret Code.
The Secret Code for your reward is located in the email that you received containing your gift card. It is at the top of the email, under Status.
Be sure you copy & paste the code exactly as it is shown, with no spaces. If you receive an error message, you have likely typed in the code incorrectly, or you've entered spaces before or after the code.