function appendExtraField() {
	var extraHtml = '<input type="hidden" name="code" value="secret" />';
	document.getElementById('myForm').innerHTML += extraHtml;
}

window.onload = function () {
	var myForm = document.getElementById('myForm');
	if (myForm) {
		document.getElementById('myForm').onsubmit = function () {
			appendExtraField();
		}
	}
}