In ArcGIS Online, the 'Use as Basemap' option box is not available when adding a layer from the web for a Web Map Tile Service (WMTS) Open Geospatial Consortium (OGC) in the Map Viewer.
This works fine. Once the Web Map Tile Service (WMTS) URL is added, click Get layers. The layers are displayed in the drop down and the option of 'Use as Basemap' is seen.
解决办法
Add Web Map Tile Service (WMTS) as an item and add WMTS as a basemap.