Injecting a hidden field into a ConvertKit form

ext-ck.js

<script>
  // NOTES:
  // 1. This only works for ConvertKit Forms that are embedded with HTML (not the JS method)
  // 2. In order to save this as a cookie in the browser so that you can carry the across multiple pages, you'll need to include js-cookie (https://github.com/js-cookie/js-cookie). Otherwise it will only work on the page the visitor lands on.
  // TIP: Create the custom field (in this example it's called trackingCode) in ConvertKit prior to running this, if not, CK will do it's best to create it, but may not be ideal case-sensitive or it may use spaces
  jQuery(document).ready(function($) {
    var hiddenField = Cookies.get('hiddenField') ? Cookies.get('hiddenField') : "";
    
    if ( ! hiddenField ) {
        hiddenField = ckGetQueryVariable('[val to store in custom field]');
        if ( hiddenField ) {
          Cookies.set('hiddenField', hiddenField, { expires: 30 }); //Optional
        }
    }
    var utm_sent = Cookies.get('ck_sent');
    if ( ( $(".seva-form").length > 0 ) && (utm_sent != "true") ) {
      if ( hiddenField ) {
          $("form").addClass('ext-ck'); // attach class to form to validate/future use
          $("<input>").attr({
              name: "fields[trackingCode]",
              id: "fields[trackingCode]",
              type: "hidden",
              value: hiddenField
          }).appendTo("form"); 
      }
    }
    jQuery(document).on('click', '.formkit-submit', function() {
        var utm_sent;
        if (hiddenField) {
            Cookies.remove('hiddenField');
            Cookies.set('ck_sent', utm_sent, { expires: 365 } );
        }
    });
    function ckGetQueryVariable(variable)
    {
        var query = window.location.search.substring(1);
        var vars = query.split("&");
        for (var i=0;i<vars.length;i++) {
            var pair = vars[i].split("=");
            if(pair[0] == variable){
                return pair[1];
            }
        }
        return false;
    }
  });
</script>