You can enter a promo code at checkout. The promotional code field is visible at the righthand side of the basket when on a laptop/ desktop/ computer. This is visible when you scroll down in the basket on mobile. For both desktop and mobile, it is below the 'Continue to Checkout' Button. You will need to enter the code in the promo code box and hit 'Apply' for the code to register.
If a promotion is automatic, but requires you to select your option, this will be visible at the bottom of your basket/ cart on both mobile and desktop.