<script>
 AJS.toInit(function (){
	var iframe = AJS.$('#checkout_monthly'); 
    var iframeSrc = 'https://app.acuityscheduling.com/catalog.php?owner=16697280&action=addCart&clear=1&id=577685&certificate=FREEMAR20&firstName=' + userFirstName + '&lastName=' + userLastName + '&email=' + userEmail; 
    iframe.attr('src' , iframeSrc); 
});
</script>
<iframe id="checkout_monthly" width="100%" height="800" frameBorder="0"></iframe>