If you mean using a custom mesh you will have to make the CK generate the default one first through FaceGen, it will provide a link to the mesh like so
You can generate it by selecting your NPC in the CK and pressing Ctrl+F4
meshes/actors/character/FaceGenData/FaceGeom/__pluginname__.esm/__formid__.nif
You might need your plugin to be a master for it to actually load the mesh. I know that you need it to be a master for face tinting to work, but you may be okay if your using just the mesh.