Change redirectUri so it use the regex version

[MargretheR]

        string redirectUri = string.Format($"http://127.0.0.1:{browser.Port}/desktop-callback");

or just
string redirectUri = string.Format($"http://127.0.0.1:{browser.Port}");

Native apps will get this default in developer portal wizard
^http://127.0.0.1\:\d{4,10}$

[SuperOfficeDevNet]

There is no file or line reference. So you mean this line?

[MargretheR]

Yes, so it's more similar to the WebApiClient login, using
// create a redirect URI using an available port on the loopback address.
// requires the OP to allow random ports on 127.0.0.1 - otherwise set a static port
var browser = new SystemBrowser();
string redirectUri = string.Format($"http://127.0.0.1:{browser.Port}/desktop-callback");