From your screenshot, it feels like your user story is “User should be able to create an EVENT which will get conducted in an MeetingRoom that is an a Zone” – correct me if i am wrong
Then first you need to have dedicated entity called zone which has Many to 1 association towards Room (Room * – 1 Zone) because 1 Zone can have many rooms (its upto you to decide)
First load the Zone entity with few zones
Then load the room entity with few room data with correct Zone selected (Zone can be selected using a ReferenceSelector)
Now in your New Event page, add a reference select over an association to Room, then add a Dropdown in which you can select the Zone via the Room relationship. That when you end up selecting a Zone you only get those Rooms which the selected Zone has.
If you dont want to have the dedicated Zone entity then leave it as an attribute in the Room Entity itself. Same procedure works there as well.
Good luck. (its kind of a long explanation but worth the try)