I’ve wondered about this for awhile. I never got it to work. The easiest way seems to be to call a function from the scene with the object as a parameter. The event would need to run on every frame or at least when needed.
Behaviors like the platform and pathfinder use 2 behaviors. They use a manager to create a list of objects. I don’t understand the manager part.
See this thread.
https://forum.gdevelop.io/t/behaviors-and-behavors/61519?u=keith_1357