You may enter a promotional code while booking your ride.
When booking on the website:
The promotional code field is located under the credit card details.
When booking on the mobile app:
The promotional code field will pop up after you click on "Book for $xx". A confirmation of your trips details will open and you will have the option to enter a code.
Please note, NEW2WINGZ and AFFPUY10 are no longer valid promotional codes.