@RobertScheitlin__GISP I think you may be looking at something different than what I am referring to?
Are you referring to the specifying the application's Redirect URI, in step #6 of the documentation for Create Client ID using ArcGIS Online or ArcGIS Enterprise? There are no validation rules on that field that prevent you from using localhost or your device name.
What you register as an app's Redirect URI, however, also has to be covered by an Allowed Origin on your ArcGIS Online instance. Otherwise, you will get the CORS error I noted above, in step #8, when you start the Experience Builder server on your local machine.
It is the Allowed Origin setting that is the issue. In ArcGIS Online, under Organization --> Settings --> Security --> Allowed origins, you can no longer add a domain that is just a machine name or localhost, such as what the documentation for Experience Builder suggests using, https://localhost:3001.