I think you mix up the Supplementary heater for diesels and the Parking heater which is an option my car have.
The Supplementary heater help the diesels cabin air to heat up faster. It works electric when the car is running and cannot preheat the cabin.
The Parking heater is a seperate unit that preheat the cabin running on fuel. It runs when a control button is pressed, via remote signal or preprogrammed in the infotainment. There is a further option to control the heater with the DFREEZE app, but then you need to add a 3G control box to the system.