I am trying to access http://[your host]/webappviewer/?config=sample-configs/config-demo.json to test a custom in-panel widget. (Create a custom in-panel widget—Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developer...) However, when I try entering the URL it just bring me to the main page of Web AppBuilder for ArcGIS. Any ideas why this is happening?
I am not sure what you are doing wrong then, because when I use this url it works fine bringing up the web app viewer:
I found the solution. The correct url is http://[your host name: port]/webappbuilder/stemapp/?config=sample-configs/config-demo.json
It can be found here: https://developers.arcgis.com/web-appbuilder/guide/get-started.htm