protected override void RegisterRequiredTypes(IContainerRegistry containerRegistry)
{
    // DelayedScopedRegionCreationBehavior
    containerRegistry.Register<DelayedRegionCreationBehavior, DelayedScopedRegionCreationBehavior>();
    
    // Shell Service
    containerRegistry.RegisterSingleton<IShellService, ShellService>();

    base.RegisterRequiredTypes(containerRegistry);
}

protected override void ConfigureDefaultRegionBehaviors(IRegionBehaviorFactory regionBehaviors)
{
    base.ConfigureDefaultRegionBehaviors(regionBehaviors);
   
    regionBehaviors.AddIfMissing(RegionManagerAwareBehavior.BehaviorKey, typeof(RegionManagerAwareBehavior));
}

public class MainWindowViewModel
{
    private IShellService shellService;
    private DelegateCommand showShellCommand;

    public MainWindowViewModel(IShellService shellService)
    {
        if (shellService is null)
            throw new ArgumentNullException(nameof(shellService));

        this.shellService = shellService;
    }

    public DelegateCommand ShowShellCommand
    {
        get
        {
            if (showShellCommand == null)
                showShellCommand = new DelegateCommand(ExecuteShowShellCommand);
            return showShellCommand;
        }
    }

    private void ExecuteShowShellCommand()
    {
        shellService.ShowShell<MainWindow>();
    }
}

public class MainWindowViewModel : IRegionManagerAware
{
    public IRegionManager RegionManager { get; set; }
}