A custom basemap cannot be toggled to in the basemap gallery of hosted web applications.
Last Published: November 30, 2015ArcGIS Online
Bug ID Number
March 11, 2015
November 4, 2023
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
Currently the basemap gallery and basemap toggle widgets do not support this function. This is a known limitation.