Just to provide help if anyone else is in this situation.
- You need to have a Native Application as well as an API registered.
- You must specify the audience in your WPF (or any native) login call. No clear documentation how how to do this in WPF, so here is what I finally found, and it works for me.
var extraParameters = new Dictionary<string, string>();
extraParameters.Add("audience", "your-audience.com");
var loginResult = await _client.LoginAsync(extraParameters: extraParameters);