How to lock you key fob in your car

I do a lot of kayaking and mountain biking. We often need to shuttle the car to the end of a ride. Typically you give an extra key to the shuttle driver. He parks the car at the take out, leaves the keys under the seat, locks the car and walks away.

I can not find a way to lock the car with the keys under the seat. Is there a way to do this?

Thanx
Ron