Solution - Working

services.AddTransient<LoginWindow>();  // When Adding Services DI


protected override void OnLaunched(LaunchActivatedEventArgs args)
 {
     var loginWindow = App.GetService<LoginWindow>();
     loginWindow.LoginButton.Click += async (sender, e) => {
         if (loginWindow.ValidLogin)              {
             base.OnLaunched(args);
             await App.GetService<IActivationService>().ActivateAsync(args);
             loginWindow.Close();
         }
     };
     loginWindow.Activate();
 }