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()).