Leaflet ESRI non-default basemap layer name

[Simons Christopher]

The Leaflet ESRI plugin denies a basemap name other than 'Streets', 'Topo', etc.  Is there a way to override this list so that the developer can specify a basemap *other* than the ArcGIS Online defaults?

In addition, the Leaflet ESRI documentation never shows an example where a basemap URL is specified; do the examples assume the user is hitting the public ArcGIS Online server?

I'm a bit new to ArcGIS but familiar with mapping APIs, such as Leaflet and Cesium, in particular.  Any 'getting started' advice with Leaflet/ESRI would be appreciated.

Thanks.