How to attach static mesh to static mesh by socket in an actor?

I can add sockets in static mesh. However, how to apply the sockets when we use the mesh as Static Mesh component in an actor?

As the following image, the StaticMesh component use the SM_DoorFrame mesh. And in the SM_DoorFrame mesh, I add two sockets as in the second picture, namely Fire and Door. However, I can’t attach StaticMesh1 component to either of the two sockets. How to do it?

Did you try following these instructions?

If that still does not work, you may try dragging StaticMesh1 out of being a child of StaticMesh and see if that gives you different results when you try to attach it.

Yes, I’ve seen that. However, the author just dragged the mesh into the world, and it becomes a static mesh actor. Now I want to do it in an actor, and I want to add a static mesh component to another static mesh component. How can I make it?

I think I’ve solved it. My actor may have a static mesh component, and its static mesh is the one I referred. What I should do is to attach the actor
to the target static mesh component. And unreal engine has a function to fulfill this, named AttachToComponent().