The ideal workflow would be: User logs into my ArcGIS Online organizational account --> finds the app item (basically a reference to the web apps URL) in a group he has access to --> gets redirected to that URL --> does NOT need to sign in again to ArcGIS Online via this dialog that opens.
I researched ESRI documentation, tried a whole lot of things related to registering apps in ArcGIS Online to get the appid etc., but can't figure it out. The Web App builder seems to be designed to provide a mechanism to do that, though, via the config.json's appid and portalurl variables. Any help would be much appreciated. Thank you!!