square.onBookingCustomAttributeOwnedUpdated subscribes to booking.custom_attribute.owned.updated for the selected seller. It does not require an additional Square OAuth permission.
Example
automations/square-on-booking-custom-attribute-owned-updated.automation.ts
Event
A custom attribute owned by your application was created or updated for a booking. The signal resolves to the officialSquare.BookingCustomAttributeOwnedUpdatedEvent payload with type narrowed to booking.custom_attribute.owned.updated.