Web app integration: popup shows "refuse to connect" error

For my web app integration I have specified an ngrok url for both Client Callback URL and Preliminary Callback URL. 

And I have set the web app integration to show a popup. 

However, The popup gives me the error: "sddaad.ngrok.io refused to connect" 

I specified sddaad.ngrok.io/teams/tabs/web as the endpoint for both Client Callback Url, and Preliminary Callback Url and just have it respond with a simple json {'message':'hello'.} (If I have it respond or redirect with HTML I still get the same error)

I don't really understand what I am supposed to respond with to send the client a GUI

I can also see a query parameter "redirect_url." It looks like: https://app.box.com/index.php?rm=box_openbox_redirect_to_box&service_action_id=20171&item_id=733018525417&item_type=file&auth_token= Am I supposed to redirect this to the user or something? 

PS: I also made sure to set sddaad.ngrok.io as an "allowed domain" in the configuration area of the app. 

Picture of error (this is what is showing up in the popup window):