MktoForms2 :: Additional form validation onValidate

HTML

<script src="//app-sj01.marketo.com/js/forms2/js/forms2.min.js"></script>
<form id="mktoForm_96"></form>
<script>MktoForms2.loadForm("//app-sj01.marketo.com", "410-XOR-673", 96,
    function(form) {   
  form.onValidate(function(){
    // Check a sample condition
    if(form.vals().LastName != "[A-Za-z0-9]"){
      //prevent form submission
      console.log('setting form unsubmittable, try again');
      form.submittable(false);
      form.showErrorMessage("Only specific text is allowed.")
    }else{
      //enable submission for those who met the criteria.
      console.log('setting submittable (submit step will run)');
      form.submittable(true);
    }
    })
    });
</script>