Open the app menu, tap Promotions, Redeem Promo Code and then Apply. The discount will be applied to your next journey. You cannot use a promo code on journeys paid for by corporate credit card.
You can see all your active promo codes in the Payment section of the app. Tap any code to view additional details such as the discount offered, the number of journeys it is valid for, and the expiry date as well as the cities, service levels and type of journey (regular or airport transfer) applicable.
Promo codes for airport transfers can only be applied if either the pickup or a drop-off location is located within official city boundaries - see Cities and Fares for more details.
If you apply multiple promo codes at once, the code granting the largest discount is activated first.