If you search around this site, you can find a few website links for pages that owners made to put on the touchscreen to explain how to stop/start car, what to do with the key, etc.
At the beginning, I always took a couple of minutes to explain to the guy taking my car how to use it, and how to shut it "off". When picking up my car, I explained to whoever was taking my ticket that the car was unusual and I needed to speak to whoever was getting the key to explain how to get in and get it started.
It doesn't always work, but I've had more success than not. Just last night I found my hood popped -- this is a frequent occurrence (I'm assuming they are hitting the key), and there is a warning on the dash when this happens. It's also good to remind people that you can track the car at anytime on your app, so you know if the car is moving, where it is, how fast it's going etc.
Good luck and congrats!
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I should also mention that with the shift stalk, valets will know how to use it because it's exactly the same as Mercedes' from 2008-2012 (they slightly changed it for 2013 models). So no worries with that -- it's more telling them not to hit the key (and accidentally open the frunk), and how to turn on and off the car (or not, as the case may be).