Put the car in what would normally be 'reverse', and leave the handbrake off.
Pushing on either the front 'A' pillar or 'B' rear pillar, rock the car backwards and forwards.
You may hear it finally drop into gear, or the car will suddenly start beeping as the gear selects gear '0'.
https://www.fq101.co.uk/how-to-guide...he-3-bars.html