public class MyCustomCredentialsAuthProvider : CredentialsAuthProvider
{
    public override bool TryAuthenticate(IServiceBase authService, string userName, string password)
    {
        if (userName == "testuser" && password == "1234")
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public override IHttpResult OnAuthenticated(IServiceBase authService, IAuthSession session, IAuthTokens tokens,
                                                Dictionary<string, string> authInfo)
    {
        session.FirstName = "Testuser Joe Doe";

        authService.SaveSession(session, SessionExpiry);
        return null;
    }

}

public partial class App : Application
{
    public JsonServiceClient ServiceClient { get; private set; }

    public App()
    {
        this.InitializeComponent();
    }
    // ....
}

private void Login()
    {
        var baseUri = $"http://{AppServer}:8088";
        ((App)Application.Current).InitServiceClient(baseUri);
        var client = ((App) Application.Current).ServiceClient;

        //var response = client.Send<AuthResponse>(new Auth { UserName = "Test", Password = "TestPassword" });
        var authResponse = client.Post(new Authenticate
        {
            provider = CredentialsAuthProvider.Name, // <-- WHAT SHOULD THIS BE???
            UserName = "testuser",
            Password = "1234",
            RememberMe = true,
        });
        // ....
    }