ASP.Net Web Forms - I want to open a popup when the user leaves the page

I am using ASP.Net web forms. My goal is to ask a user a question when leaving the page. I tried using window.onbeforeunload for this. But in onbeforeunload the popup cannot be customized. I want to use my own popup too. The popup I created opens successfully. But after the popup is opened, the page closes immediately. Note: I'm trying to do it with Javascript. How do I resolve this situation?

var unSavedStatus = false;

window.onbeforeunload = SaveControl;

function SaveControl()
{
  if(unSavedStatus)
    myPopup.Show();  
}