// JavaScript Document
function fireMyPopup() {

	var scrolledX, scrolledY;
if( self.pageYOffset ) {
  scrolledX = self.pageXOffset;
  scrolledY = self.pageYOffset;
} else if( document.documentElement && document.documentElement.scrollTop ) {
  scrolledX = document.documentElement.scrollLeft;
  scrolledY = document.documentElement.scrollTop;
} else if( document.body ) {
  scrolledX = document.body.scrollLeft;
  scrolledY = document.body.scrollTop;
}

// Determine the coordinates of the center of the page

var centerX, centerY;
if( self.innerHeight ) {
  centerX = self.innerWidth;
  centerY = self.innerHeight;
} else if( document.documentElement && document.documentElement.clientHeight ) {
  centerX = document.documentElement.clientWidth;
  centerY = document.documentElement.clientHeight;
} else if( document.body ) {
  centerX = document.body.clientWidth;
  centerY = document.body.clientHeight;
}
  
  // ABOVE CODE GOES HERE
  var leftOffset = scrolledX + (centerX - 900) / 2;
var topOffset = scrolledY + (centerY - 1000) / 2;
  //END ABOVE CODE GOES HERE

  document.getElementById("mypopup").style.top = topOffset + "px";
  document.getElementById("mypopup").style.left = leftOffset + "px";
  document.getElementById("mypopup").style.display = "block";
}