Does any know how to use this app? When I run it, a blank screen comes up, pressing the middle watch button displays "HTTP Error, Status code: 403.
Error Code 403 is the Forbidden Error. This simply means that access to the page or resource is forbidden. Clearing the cache files fixes the issue sometime. You should try clearing the data of the app. Try logging in and then try to access the specific page. Still could not fix the issue iCloud Support will help you to find the proper answer.
After working on it some more, I was able to get the app to display an authorization code on the watch. However, the GetHub link on my phone did not get me anywhere.