Add ESRI Dynamic Map Services (MapServer)
Example MapServer Layers:
https://carto.nationalmap.gov/arcgis/rest/services/transportation/MapServer
https://carto.nationalmap.gov/arcgis/rest/services/transportation/MapServer
https://hydro.nationalmap.gov/arcgis/rest/services/nhd/MapServer
Do not include any query parameters or extra mapserver URL information. Other than if you are only loading one layer then that layerid