It would be very useful to be able to load a GeoJSON file from an external source (API) that requires basic authentication.
Currently, when you access such a GeoJSON file via a web browser, a pop-up window prompts you to enter a username and password :

However, when loading the URL into a web map or as a new item in ArcGIS Online, authentication is not possible.

The use case is simple: to be able to display and refresh vehicle positions in a web map from a GeoJSON API provided by an external vendor without having to use a proxy for authentication or a tool like Velocity.