You need to be registered in the Mercedes-Benz /developer portal for the vehicle images product.
https://developer.mercedes-benz.com/products/vehicle_images/

Go to “GET ACCESS”, choose test this product and select one of the trial options (placeholder, basic or 360).
The trial products covers a limited number of free calls.
Than you can use your apikey and pass it to the RapidAPI request.

The API is still in POC mode, but should work. We are open to get your feedback (developer@daimler.com)