On the website: Go to "account" and then scroll down! There will be a black bubble that says "add promo or gift card!"
On the app: Click the three bars on the upper left hand side of the screen. Go to "enter promo/gift card" and enter the code!
Once the promo code has been added, simply ensure that you tap on “redeem” at the checkout screen to apply your discount. Your total will reflect accordingly.