In the Beam app, and during a trip, you will find buttons named "Tap to lock vehicle" and "Tap to unlock vehicle" available for use. Use these features to temporarily lock and unlock your Beam vehicles as necessary.
For example, when you need a toilet break during a ride, park your vehicle in a safe place and use "Tap to lock vehicle" in the app to prevent other riders from using it. When you return to the Beam vehicle, open the app and use "Tap to unlock vehicle" to unlock the Beam vehicle and continue riding to your destination.
However, note throughout the duration your Beam vehicle is locked, you will still be charged for every minutes for occupying the Beam vehicle. If you have any active Ride Pass or subscription, the benefits will be utilised instead.