Capture the UTM values from a querystring and store them in a long lived document cookie.

cookie2form.js

// Append UTM values to any form on the page
(function () {
  function getUTMCookies() {
    var cookies = {}, pairs = document.cookie.split(";"), i, pair, key;

    for (i = 0; i < pairs.length; i++) {
      pair = pairs[i].split("=");
      key = (pair[0] + '').trim();
      if (key.indexOf('utm') > -1) {
        cookies[key] = unescape(pair[1]);
      }
    }

    return cookies;
  }

  function appendUtmToForm() {
    var fields = [];
    $.each(getUTMCookies(), function (index, value) {
      fields.push('<input type="hidden" name="' + index + '" value="' + value + '" />');
    });

    $('form').append(fields);
  }

  // Grab the cookie value and set the form field values
  appendUtmToForm();
}());

utm2cookie.js

(function () {
  var query_string, pieces, hash, key, i, value, exp_date;

  function setYearLongCookie(cname, cvalue) {
    var expires = "expires=" + exp_date.toUTCString();
    document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
  }

  query_string = window.location.search.substr(1);

  if (!query_string) {
    return;
  }

  exp_date = new Date();
  exp_date.setTime(exp_date.getTime() + (365 * 24 * 60 * 60 * 1000));

  pieces = query_string.split('&');
  for (i = 0; i < pieces.length; i++) {
    hash = pieces[i].split('=');
    key = hash[0].toLowerCase();
    value = hash[1];
    if (key.indexOf('utm') > -1 && value) {
      setYearLongCookie(key, value);
    }
  }
}());