WPF popup window

You need to create a new Window class. You can design that then any way you want. You can create and show a window modally like this:

MyWindow popup = new MyWindow();
popup.ShowDialog();

You can add a custom property for your result value, or if you only have two possible results ( + possibly undeterminate, which would be null), you can set the window's DialogResult property before closing it and then check for it (it is the value returned by ShowDialog()).

In WPF there is a control named Popup.

Popup myPopup = new Popup();
//(...)
myPopup.IsOpen = true;