This is a modification of the Overview Map sample on ArcGIS Developers: ArcGIS Maps SDK for JavaScript Sample Code | Overview Map
The default app uses a webmap with the Equal Earth projection in the main view and uses the Light Gray Basemap for the overview globe. Example
<base URL>?webmap=<webmap ID>
where <webmap ID>
is any ArcGIS Online webmap item.
Example: https://davidasbury.github.io/scene-overview-map/?webmap=22d1524dde5f4e5b9670545f5b853517
The main map can use any projection.
<base URL>?webmap=<webmap ID>&overview=<webmap ID2>
where <webmap ID2>
is any ArcGIS Online webmap in WGS84 or Web Mercator projection. This is a requirement of the 3D SceneViewer.