In order to work, the Open box needs to be somewhere with an internet coverage so your car should not be parked in an underground parking. Also, the car needs to be parked somewhere accessible on foot by the driver, on their own.
At the end of the rental, the driver should park the car where they found it, if possible. If you don't have a reserved parking space, don't forget to fill out the return instructions: this is where you'll be able to tell the driver where to leave the car. To add your return instructions, go to Your Cars > Manage your listing > Rental instructions.