where is the freon Charging point for the A/C system? on 1991 Mercedes-Benz 560SEC

Hot air is blowing suddenly I wish to charge the coolant gas but I am not able to pin point the inlet point

Asked by for the 1991 Mercedes-Benz 560SEC
The low side service port is located to the left of the left cylinder head on the A/C line above the front spark plug.