On the map screen, below the status of the service, vehicle type, and payment method, you will be able to activate/deactivate the tipping (10%, 15%, 20%). Also, inside the payment method menu, you can select the tip amount and set it by default for upcoming trips.
You can tip your driver just before starting or during the service. Once the trip is over, it's no longer possible.
Important: Tipping can only be set from the passenger's app. Neither FREENOW nor the driver can determine this contribution.