public partial class App : Application
{
    public App()
    {
        this.InitializeComponent();

        Init();
    }

    /// Add this method and call in the constructor after this.InitializeComponent();
    void Init()
    {
        // Recommended
        RxApp.DefaultExceptionHandler = new SextantDefaultExceptionHandler();
        Locator.CurrentMutable.RegisterViewsForViewModels(Assembly.GetCallingAssembly());
        
        // Required
        Locator.CurrentMutable
            .RegisterWinUIViewLocator()
            .RegisterParameterViewStackService()
            .RegisterViewStackServiceFromParameterService()
            .RegisterNavigationView()
            .RegisterConstantAnd<IDialogManager>(new DialogManager());
        // Will be expanded later...
    }
}

public partial class App : Application
{
    public App()
    {
        this.InitializeComponent();

        Init();
    }

    // Add this code block

    public IServiceProvider? Container { get; private set; }

    /// Call this method in the constructor after this.InitializeComponent();
    void Init()
    {
        RxApp.DefaultExceptionHandler = new SextantDefaultExceptionHandler();
        var host = Host.CreateDefaultBuilder()
            .ConfigureServices(
                services =>
                {
                    services.UseMicrosoftDependencyResolver();
                    var resolver = Locator.CurrentMutable;
                    resolver.InitializeSplat();
                    resolver.InitializeReactiveUI();

                    // Configure our local services and access the host configuration
                    ConfigureServices(services);

                    // Configure Sextant, Views and ViewModels
                    services.UseSextant(
                        builder =>
                        {
                            builder.ConfigureDefaults();
                            builder.ConfigureViews(
                                viewBuilder =>
                                {
                                    // Will be expanded later...
                                }
                            );
                        }
                    );
                }
            )
            .UseEnvironment(Environments.Development)
            .Build();

        // Since MS DI container is a different type,
        // we need to re-register the built container with Splat again
        Container = host.Services;
        Container.UseMicrosoftDependencyResolver();
    }

    void ConfigureServices(IServiceCollection services)
    {
        // register your other services here
    }
}

// Tell navigation service which <Frame /> to use
// A reference to the containing <Window /> is also needed for Popups.
_navigationService.SetAdapter(new FrameNavigationViewAdapter(MyFrame, MyWindow));

// Tell navigation service which <Frame /> && <NavigationView /> to use
// A reference to the containing <Window /> is also needed for Popups.
_navigationService.SetAdapter(
    new NavigationViewAdapter(MyFrame, MyWindow, MyNavigationView)
);

Locator.CurrentMutable
    .RegisterViewWinUI(
        () => new MyView(),
        () => new MyViewModel()
    );

viewBuilder.RegisterViewAndViewModel<
    MyView,
    MyViewModel
>();