There appear to be several things wrong with this scenario, but I’m probably mistaken
Somehow, someway, you actually left your car keys so far away from the car that you had to order a new key from a dealer. You couldn’t have someone use UPS, FedEx, or “Joes Key Delivery Service” to overnight you “your key” for “your car”, you had to hand over several hundred dollars to a Mercedes-Benz dealer. Forget the fact that the dealer is going to want to see registration papers and a photo ID proving that you own the car, and that most people keep those registration papers locked in the car that you don't have a key for. I’ll just assume the car isn’t locked, you have the papers in hand, and you managed to drive to the dealer (without a car) and order the key. So far, so good, right? The problem is - the dealer won't supply a loaner, you need the car tomorrow AND you would like someone on a public forum to explain to you how to hotwire a Mercedes-Benz. And it's your first post.