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API Authentication with PKCE for desktop app

Started by Neville Clarke -   in API Authentication

I am updating a legacy desktop app (written in VB) to use OAuth2. The example PKCE shown at https://github.com/XeroAPI/net-desktop-pkce-example was used to prove that a valid connection could be made using a callback of 'http://localhost:46300/callback'
However when incorporating into the legacy app I cannot get the listener to work. The browser shows the xero request to allow access but after clicking allow the listener does nothing.
Fiddler shows the response code, scope and session_state but result code 504.
netstat -ano shows listening on local address port 46300 rather than port 46300

Because I was unable to get a VB version of the listener to work I then decided to add a C# project containing the class library https://github.com/AGenius/XeroAuth2API to the VB solution.
This also results in the same code 504.

Can anyone suggest where to look to get this listener working?