To add a promo code to your Uber order
Open the Uber app.  Tap the History button.  Tap Payment.  Tap Add Promo/Gift Code.  Type in your PROMO CODE.  Tap ADD.
Open the Uber app.  Tap the menu button.  Tap Payment.  Tap Add Promo/Gift Code.  Type in your PROMO CODE.  Tap ADD.