ClientApiPoC/ApiClient/App.xaml.cs

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System.Windows;

namespace ClientApiPoC.ApiClient {
    public partial class App : Application {
        private IHost _host;

        public App() {
            _host = Host.CreateDefaultBuilder().ConfigureServices(services => {
                // ViewModels:
                services.AddTransient<MainWindowViewModel>();
                // Views:
                services.AddSingleton<MainWindow>();
            }).Build();
        }

        protected override async void OnStartup(StartupEventArgs e) {
            await _host.StartAsync();

            var window = _host.Services.GetRequiredService<MainWindow>();
            window.Show();

            base.OnStartup(e);
        }

        protected override async void OnExit(ExitEventArgs e) {
            await _host.StopAsync();
            _host.Dispose();
            base.OnExit(e);
        }
    }
}