internal static class InteropExtensions
{
    public static bool? ShowDialog(this System.Windows.Window win, IntPtr handle)
    {
        WindowInteropHelper helper = new WindowInteropHelper(win);
        helper.Owner = handle;
        return win.ShowDialog();
    }
}

var win = new WpfWindow();
win.ShowDalog(windowsFormOwnerHandle);

public VariablesWindow()
  {
   this.InitializeComponent();
   variables.AddHandler(Button.ClickEvent, new RoutedEventHandler(HandleChildClick));
  }
  
  private void HandleChildClick(object sender, RoutedEventArgs args)
        {
            Button senderButton = args.OriginalSource as Button;
            if (senderButton != null && senderButton.Content != null)
            {
                string buttonText = senderButton.Content as string;
                if (buttonText == "_Cancel")
                {
                    this.DialogResult = false;
                    Close();
                }
                else if (buttonText == "_OK")
                {
                    this.DialogResult = true;
                    Close();
                }
            }
        }