Let's say you called your popup window class "PopupWindow". You created it by right-clicking on your project in project explorer and chosing "Add new Windows Form". Then you added a text box and an "Ok"-Button.
You also added a property to your form so that you can later gain access to the data the user entered in the text box.
public class PopupWindow
public string EnteredText
return( textBox.Text );
Now you use your new class like this:
PopupWindow popup = new PopupWindow();
string userEnteredText = popup.EnteredText;