The option to enter a promo code will only present itself if the cards in your cart are from the "Discounted Gift Card" section of the brand's Buy Page. If you have selected a card from the "CashBack Gift Cards" section, you will not see an option for promo code entry.
Once you have confirmed that the cards in your cart are discounted cards, you can apply it on the "Secure Checkout" page. Your order summary on the right hand side has a "Use a promo code" option. Select this feature and enter your code. Click "Apply", submit your order, and just like that you've saved some extra cash!
If you prefer to shop with the Raise app, you can enter a promo code in your cart before submitting your order—just tap “Have a promo code?” and apply.