C# WebBrowser -> WebView2 solution

Here's some other info that may be useful. From my reading (not testing,) WebBrowser, available in .NET, runs in IE7 mode by default. There are some reg keys you can use to push it up to IE11 but MYOB have said even that won't be supported soon, so switching to WebView2 makes sense.

After you make the original GET request, after a lot of bouncing around (about 5 calls, which you can see if you put a break on webView2.NavigationStarting in my code) you end up with a return result that includes your chosen redirect URL, e.g. http://desktop, and a bunch of query parameters that include the code you need to POST to get the access tokens. I'm guessing that 'bouncing' now won't happen if you're not using a modern browser (maybe the issue is javascript related?)

The return URL (if using http://desktop) will definitely result in a navigation error because it doesn't point to a valid resource, but is serves the purpose of presenting the code, which you can extract for the next step. I don't remember seeing a page navigation error with the WebBrowser control, probably because the form was closed before it was rendered. With my code, you do get a flash of the error page before the form automatically closes.

I guess if you can't find suitable browser to invoke within your app, you could start an external browser to make the GET request, and make the redirect URL something like http://localhost:1234 and set up a port listener in your app to retrieve the result. Or if that's also a problem, simply provide instructions for the user to copy and paste the redirect URL back into your app so you can extract the code.