Is it possible to add a basemap which is a WMS service to 3D scene? It seems that only WMTS is supported. I am getting an error "Unsupported basemap layer type WMS". I know that I can add WMS to operational layers and then use it in the scene, but I would prefer it if it is defined in the basemap gallery and used from there.
I believe you will need a tile layer as well to drive the tiling scheme of the scene if a basemap is present.
Quick example of a WMS and tile layer as a basemap in a web scene in Scene Viewer https://arcg.is/1qePX4
I am not authorized to see the web scene.