Hello,
I think it would be good to add support for tileServers for basemap defined in AGOL or more generally in the webappconfig json.
As it seems that it cannot be done on the server side contrary to what ESRI REST API documentation state:
Define tile servers of tiled map services
ArcGIS Server REST API
So far it can only be used with defining an ArcGISTiledMapServiceLayer :
ArcGISTiledMapServiceLayer | API Reference | ArcGIS API for JavaScript 3.19
So it cannot be easily integrated in the WebAppBuilder.
Thanks,
Nicolas