Go to the menu and tap on "Credit & Vouchers". You can see your active vouchers on this page. These will automatically be used as payment method for your next booking.
If you have received a promo code, you can enter it here by tapping on the "Redeem Voucher" button and the credit will be integrated in the app. The credit will be used and credited immediately with the next booking. If you make a booking with less than the credit value, you will receive a €0 invoice.
Which one is used first for payment?
The voucher that expires next is always used first.
Can more than one voucher be used for the same payment?
If a payment exceeds the value of a voucher, the next voucher will automatically be used for the remaining amount. Only if there are no more vouchers available for payment will your stored payment method be charged.