Add component to actor in C++ (the final word!)

Well it does this:

void AActor::AddInstanceComponent(UActorComponent* Component)
{
	Component->CreationMethod = EComponentCreationMethod::Instance;
	InstanceComponents.AddUnique(Component);
}

Sets the CreationMethod to Instance and adds it to the InstanceComponents set which is:

Array of ActorComponents that have been added by the user on a per-instance basis.