Connect to mobile.de search API and prepopulate WP Gravity Forms fields with API parameters

I have "search API" access for [login to view URL] ([login to view URL]). It shows all information for the cars of a car seller. Task is to create API connection and to write a function (all in Wordpress) which does: - Fill-in a dropdown field in an existing Gravity Form with all ads (cars) of the API owner - When a car in the dropdown is selected prepopulate / fill-in 5 other fields in the Gravity Form with the information of the selected car (e.g. make+model, registration date, mileage, car-id)