<a href="#" id="lightbox-link">CONTACT US</a>
<script src="//app-xxx.marketo.com/js/forms2/js/forms2.js"></script>
<form id="mktoForm_618"> </form>
<script>
var btn = document.getElementById("lightbox-link");
btn.onclick = function(){
MktoForms2.loadForm("//app-xxx.marketo.com", "xxx-xxx-xxx",618, function (form){
MktoForms2.lightbox(form).show();
});
};
</script>