How to add basemap to list of basemaps in EB

[USLEOperations]

I need to add as a basemap, to the list of basemaps offered in the basemap widget, the following shown below.   What file(s) do I need to edited in the Experience Builder source to accomplish this?

[JeffBigos]

Experience Builder and additional configurable apps that use or have Basemap Gallery widget, will refer to the basemap group setting defined by an Organization administrator:
Basemap Gallery

To modify the choices available, a group can be created in your organization and different basemaps can be added or removed from that group to facilitate the choices you want to provide.

Note this change would apply to other apps that use this configuration to present basemaps within the organization, (mapviewer, Web App Builder basemap gallery widget)