function fireMyPopup() {

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;
}

  var leftOffset = 0 + (centerX -520) / 2;
  var topOffset = 30;

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