<script type="text/javascript">
// Parse the URL
function getParameterByName(name) {
    name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
    var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
    results = regex.exec(location.search);
    return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}
// Give the URL parameters variable names
var source = getParameterByName('utm_source');
var medium = getParameterByName('utm_medium');
var campaign = getParameterByName('utm_campaign');
 
// Put the variable names into the hidden fields in the form. selector should be "p.YOURFIELDNAME input"
document.querySelector("p.source input").value = source;
document.querySelector("p.utm_medium input").value = medium;
document.querySelector("p.utm_campaign input").value = campaign;
</script>