Basicaly, you will have as many sub meshes as geometries in your modeling tool. I don’t know how to specify a name for the submeshes, it depends on the tool you use.
At worst, you only have to find it manually. In jmonkey, the model will be created as a Node, with each submesh as a child Geometry. Just try until you find the one you’re looking for, and record the index.
The result here