To register a function using AddOnActorSpawnedHandler, you first need to create the delegate. So something like:
void MyClass::RegisterSpawnHandler(UWorld* InWorld)
{
FOnActorSpawned::FDelegate ActorSpawnedDelegate = FOnActorSpawned::FDelegate::CreateUObject(this, &MyClass::OnActorSpawned);
InWorld->AddOnActorSpawnedHandler(ActorSpawnedDelegate);
}
void MyClass::OnActorSpawned(AActor* InActor)
{
//do something
}
Although you might want to make the ActorSpawnedDelegate a class variable, so you can also remove it later (via World->RemoveOnActorSpawnedHandler). Also the :FDelegate::CreateUObject will vary depending on what type of class your function is in. So this example is in a class that is derived from UObject.