Use Delegate With Blueprint

Firstly, declare it with dynamic multicast:

	DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FElevatorReadyToGoSignature, int, ElevatorIdx);

Then, make sure to use BlueprintAssignable in the specifier:

UPROPERTY(BlueprintAssignable, Category = "Elevator")
		FElevatorArrivalSignature ArrivalDelegates;

I made a full example with my Elevator System.