Car Rental Discounts and Promotions | Enterprise Rent-A-Car If you are already in a rental vehicle, we are unable to add any discounts to the rate you have. I booked my reservation online and didn't know where to add the discount code; can it still be added? If you have not picked up a rental vehicle yet and have a reservation, please contact our reservation center at 1 (855) 266-9289 and we will be ...