I found a way different from the course. My problem was, that my object wasn’t one single object, so I couldn’t ID it and mask it.
Here’s a screenshot of my solution. (Auto = Card, Shatten = Shadow)
So my approach (also easier in more complex scenes I guess) is to isolate the shadows in the render layer where it is cast and multiply it with the background. Since it’s multiplied with the background there’s no need to mask anything since the Car model is set on top afterwards.
Of course you could also multiply the shadow at the end, but then you would need to set up the render layer that it’s masked by the other layer(s).
Here’s the result by the way. I also added the background picture as an enviroment texture to have a reflection on the car.
