When you're making an online booking, you'll be sent a confirmation email, where you'll find information about your booking including the arrival time, type of parking (shuttle- or valet) and the Parkos phone number and the phone number of the car park. This email should be printed and brought with you on the day of your departure.
If you can't print the confirmation email, then you can always show them the confirmation email on your laptop, tablet, or mobile phone. This confirmation also serves as proof for the parking provider that you have made a reservation.