Alarm* - remote control key not working

If the alarm cannot be deactivated with the remote control key, e.g. if the key's battery is discharged - the car can be unlocked, disarmed and the engine started as follows:
Open the left-hand front door with the detachable key blade.
The alarm is triggered, the direction indicators flash and the siren sounds.
P4-1220-Y55X-Remote key in ignition slot
Insert the remote control key in the ignition switch.
The alarm is deactivated.
  1. *Option/accessory.