Xamarin.Forms UITesting set App variable to mocked class before load

I am trying to declare and mock a local interface that interacts with a cloud authentication API. My local class implements this interface and provides the functionality to interact with the cloud API. To be able to run UI tests on this application, I would like to be able to implement a 'Mock' interface and inject this into the application to be used in place of the functioning class. I am currently using the Moq library.

My LoginPage currently is initialized and displayed within the App() constructor method as a navigation page. I would like to be able to either set the instance of the AuthApi as the 'MockedAPI' within my test, or inject the mock into the LoginPage directly for use. I was hoping that this 'MockedAPI' would be able to override this App.AuthApi before the application started, but it doesn't seem to be the case here.

App.xaml.cs

public partial class App
{
    public static IAuthService AuthApi { get; set; } = new AWSCognito(); // AWSCognito implements IAuthService

    public App()
    {
        InitializeComponent();
        MainPage = new NavigationPage(new LoginPage());
    }
}

LoginPageTest.cs

    [SetUp]
    public void BeforeEachTest()
    {
        var mocker = new Mock<IAuthService>();

        mocker.Setup(x => x.SignIn(It.IsAny<string>(), It.IsAny<string>())).Returns(() => new MockAuthService().SignIn("a", "a"));
        App.AuthApi = mocker.Object;
        AppManager.StartApp();

        app = AppInitializer.StartApp(platform);
        }

    [Test]
    public void EnterUsernameAndPassword()
    {
        app.WaitForElement(c => c.Marked("Welcome"));
        app.EnterText(c => c.Marked("Username"), new string('a', 1));
        app.EnterText(c => c.Marked("Password"), new string('a', 1));

        app.Tap("Login")
        bool state = app.Query(c => c.Class("ProgressBar")).FirstOrDefault().Enabled;

        Assert.IsTrue(state);
    }

MockAuthService

public class MockAuthService : IAuthService
{
    public Task<AuthenticationContext> SignIn(string username, string password)
    {
        return Task.FromResult(new AuthenticationContext(CognitoResult.Ok)
        {
            IdToken = "SUCCESS_TOKEN",
        });
    }
}

LoginPage.cs

XamlCompilation(XamlCompilationOptions.Compile)]
public sealed partial class LoginPage
{
    private string _username;

    private string _password;

    private LoginPageViewModel _viewModel;

    public LoginPage()
    {
        InitializeComponent();

        _viewModel = new LoginPageViewModel();

        BindingContext = _viewModel;
    }

    private void LoginButtonClicked(object sender, EventArgs args)
    {
          _username = UsernameEntry.Text;
          _password = PasswordEntry.Text;
          LoginToApplication();
    }

    public async void LoginToApplication()
    {
          AuthenticationContext context = await App.AuthApi.SignIn(_username, _password);
    }
}