Add Host, service

App.xaml.cs

public partial class App
    {
        public static bool IsDesignMode { get; private set; } = true;

        private static IHost __Host;

        public static IHost Host => __Host ??= Program.CreateHostBuilder(Environment.GetCommandLineArgs()).Build();

        protected override async void OnStartup(StartupEventArgs e)
        {
            IsDesignMode = false;
            var host = Host;
            base.OnStartup(e);

            await host.StartAsync().ConfigureAwait(false);
        }

        protected override async void OnExit(ExitEventArgs e)
        {
            base.OnExit(e);

            var host = Host;
            await host.StopAsync().ConfigureAwait(false);
            host.Dispose();
            __Host = null;
        }

        public static void ConfigureServices(HostBuilderContext host, IServiceCollection services) => services
               .RegisterServices()
               .RegisterViewModels();

        public static string CurrentDirectory => IsDesignMode 
            ? Path.GetDirectoryName(GetSourceCodePath()) 
            : Environment.CurrentDirectory;

        private static string GetSourceCodePath([CallerFilePath] string Path = null) => Path;
    }

Project.cproj

 <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="3.1.5" />
  </ItemGroup>

Registrator.cs

using Microsoft.Extensions.DependencyInjection;

internal static class Registrator
    {
        public static IServiceCollection RegisterServices(this IServiceCollection services)
        {
            services.AddSingleton<IService, Service>();
            
            //services.AddTransient<IService, Service>();
            //services.AddScoped<IService, Service>();
            
            return services;
        }
        
        public static IServiceCollection RegisterViewModels(this IServiceCollection services)
        {
            services.AddSingleton<MainWindowViewModel>();
            
            //services.AddTransient<MainWindowViewModel>();

            return services;
        }
    }