Subscribing to the event StartPlaceMode to enable the Augmented Reality placement mode functionality.

AnchorPositioning.js

// Listen for the world event "StartPlaceWorld". This will be sent
// by another component in the scene when the placement mode should
// be active. When that event is received, this script sets placeMode to true,
// which actually allows this script to react to touch events.
this.startPlaceWorldEvent = ctx.world.event('StartPlaceWorld');
this.startPlaceWorldEvent.listen(ctx, data => this.placeMode.set(true));