Solution to using UTM parameters & cookies to capture lead source information. See https://nation.marketo.com/thread/24866

marketo_example.html

<script type="text/javascript">
// getQueryParams
//This grabs the UTM parameters from the URL
function getQueryParams(qs) {
    qs = qs.split("+").join(" ");
    var params = {}, tokens,
    re = /[?&]?([^=]+)=([^&]*)/g;
    while (tokens = re.exec(qs)) {
      params[decodeURIComponent(tokens[1])] = decodeURIComponent(tokens[2]);
    }
    return params;
}

// getQueryBoolParams
// Handles getting boolean parameters
// See https://regex101.com/r/QFnVPM/1
function getQueryBoolParams(qs) {
    qs = qs.split("+").join(" ");
    var params = {}, tokens,
    re = /[?&]([^=]+)&|&([^=]+)&?/g;
    while (tokens = re.exec(qs)) {
      params[] = decodeURIComponent(tokens[1]);
    }
    return params;
}

// checkForUTMDataInForm
// Handles checking to see if the UTM hidden fields exist on the page yet
function checkForUTMDataInForm(query, totalChecks = 100){
  var utmInt = setInterval(doCheck, 1000);
  var counter = 0;
  function doCheck() {
    // Kill the interval after total checks reached
    if (counter == totalChecks) {
       clearInterval(utmInt);
    }
    // Check to see if the utm_source exists as a test
    if( document.hasOwnProperty("utm_source") ){
      clearInterval(utmInt);
      
      //This gets the cookies
      //ID = MARKETO FIELD NAME
      addUTMData(query);
    }
  }
}

// addUTMData
// Handles adding the UTM data based on the query params
function addUTMData(query){
  // Crate an array
  var elements = [];
  // UTM Source
  if(document.hasOwnProperty("utm_source")){
    var utmSource = document.getElementById("utm_source");
    elements["utm_source"] = utmSource;
    elements["utm_source"].value = query.utm_source;
  }

  // UTM Campaign
  if(document.hasOwnProperty("utm_campaign")){
    var utmCampaign = document.getElementById("utm_campaign");
    elements["utm_campaign"] = utmCampaign;
    elements["utm_campaign"].value = query.utm_campaign;
  }

  // UTM Medium
  if(document.hasOwnProperty("utm_medium")){
    var utmMedium = document.getElementById("utm_medium");
    elements["utm_medium"] = utmMedium;
    elements["utm_medium"].value = query.utm_medium;
  }

  // UTM Medium
  if(document.hasOwnProperty("utm_term")){
    var utmTerm = document.getElementById("utm_term");
    elements["utm_term"] = utmTerm;
    elements["utm_term"].value = query.utm_term;
  }

  // UTM Content
  if(document.hasOwnProperty("utm_content")){
    var utmContent = document.getElementById("utm_content");
    elements["utm_content"] = utmContent;
    elements["utm_content"].value = query.utm_content;
  }
  
  return elements;
}

// populateCookie
// Handles populating the cookie
// Setting domain and path attributes allow cookies to be read across subdomains
// To have a cookie available to all subdomains, you must put a . in front of your domain.
// Setting the path=/ will have the cookie be available within the entire specified domain.
function populateCookie(domains, query, queryBools){
  if (!domains) {
    console.log('ERROR: You did not provide any domains.');
    return false;
  }
  var queryContainsBools = queryBools ? true : false;
  // Don't forget to change "example.com" to your domain.
  if (document.hasOwnProperty('cookie')){
    // Traverse through keyed params
    for (var key in query) {
      //console.log(key, query[key]);
      document.cookie = key + "=" + query[key];
      // Traverse through the domains
      /*
      for(var c=0;c<domains.length;c++){
        document.cookie = key + "=" + query[key] + ";domain=."+domains[c]+";path=/";
      }
      */
    }

    if (queryContainsBools){
      // Traverse through bool params
      for (var i=0; i<queryBools.length;i++) {
        //console.log(key, query[key]);
        // document.cookie = queryBools[i] + "=" + "true"; // Alternate assignment
        document.cookie = queryBools[i];
        // Traverse through the domains
        /*
        for(var d=0;d<domains.length;d++){
          document.cookie = queryBools[i] + "=" + true + ";domain=."+domains[d]";path=/";
        }
        */
      }
    }
    return document.cookie;
  }

  return false;
}

// Handles filling the cookie with content
//This sets the cookies
var query = getQueryParams(document.location.search);
var queryBools = getQueryBoolParams(document.location.search);
var domains = [ "example.com", "example2.com" ];
populateCookie(domains, query, queryBools);
 
//This runs the script so the hidden fields are automatically populated.
var totalChecks = 100;
checkForUTMDataInForm(query, totalChecks);

</script>