Just put the following below your form. Make sure your input fields are required
.
<script>
var forms = document.getElementsByTagName('form');
for (var i = 0; i < forms.length; i++) {
forms[i].noValidate = true;
forms[i].addEventListener('submit', function(event) {
if (!event.target.checkValidity()) {
event.preventDefault();
alert("Please complete all fields and accept the terms.");
}
}, false);
}
</script>