There is a 13 digit code to redeem the gift card and e-gift card against a COOK food order. This can be found in the email for the e-gift card or if you have a physical gift card, it is located on the reverse and is a long string of letters and numbers. Just remember to scratch off the panel to reveal the final section of number and letters on the back of the gift card.
At the checkout page, you can type any gift card codes you have into the Add Gift Card/E-Gift card box on the right hand side of the page and then ensure you press add.