To insert your promo code, all you need to do is:
Go to your online shopping cart once you have selected your item(s).
Click on "Pay my order".
On your smartphone, scroll down the "View my order and enter the promo code" tab, enter your code in the "discount code" box and apply. On your PC, enter your promo code in the order summary on the right of your screen in the "discount code" box and then apply.