basically you need static mesh components under your root static mesh, create a new editable Static Mesh variable, and then set those sub-static meshes in the construction script. You dont even need to set a mesh at first if you like and then set the mesh when you place the actor into the level. Just make sure the socket to attach is set in the details panel of the static mesh.
Then you’ll see the name and slot of the variable you created when you place the actor into the level.