public partial class MainWindow
{
    IRegionManager _regionManager;
    public MainWindow()
    {
        InitializeComponent();
        _regionManager = ServiceLocator.Current.GetInstance<IRegionManager>();
        RegionManager.SetRegionManager(ContentRegion, _regionManager);
        Loaded += MainWindow_Loaded;
    }

    private void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        _regionManager.RequestNavigate("ContentRegion", "ViewA");
    }
}