You can type a short message that your driver will see when he or she accepts your journey. After entering your pickup address, tap "Prebook / Options", then tap "Booking Options".
From here, you can use the "Message to driver" field at the beginning of the page to enter any relevant information. This message is sent to the driver with your order. You can call your driver before he/she arrives if there is something important you need to communicate to him or her.
To call the driver, simply tap on the telephone symbol.