Pass Tracking Parameters to a Form on Another Page Using GTM – https://zackphilipps.dev/posts/store-gclid-cookie-send-to-hubspot/

assignTrackingParameterToCookie.js

// Pass Tracking Parameters to a Form on Another Page Using GTM
// https://zackphilipps.dev/posts/store-gclid-cookie-send-to-hubspot/

/**
 * Assigns the supplied URL parameter to a cookie and each form field with a name that matches.
 * You can keep calling this function multiple times for each URL parameter you want to pass along, e.g.
 *
 * window.onload = function () {
 *   assignTrackingParameterToCookie("gclid", "hubspot");
 *   assignTrackingParameterToCookie("utm_source", "gform");
 *   assignTrackingParameterToCookie("<YOUR_UTM>", "<YOUR_FORM_TYPE>");
 * };
 *
 * @param {string} fieldParam - The name of the URL parameter. In this case, `gclid`.
 *                              Could also be any UTM parameter such as `utm_source`.
 *                              MUST match the input name (HubSpot)
 *                              or one of its CSS classes (Gravity Forms).
 * @param {string} formType - The type of form. Currently only supports 'hubspot' (the default)
 *                            or 'gform'.
 */
function assignTrackingParameterToCookie(fieldParam, formType = 'hubspot') {
  var field = getParam(fieldParam),
    inputs

  if (field) {
    setCookie(fieldParam, field, 365)
  }

  /**
   * Add other form types / input query selectors here.
   */
  if (formType === 'gform') {
    inputs = document.querySelectorAll('.' + fieldParam + ' input[type="text"]')
    assignCookieValueToFormInput(fieldParam, inputs)
  } else if (formType === 'hubspot') {
    inputs = document.querySelectorAll('.hs-input[name="' + fieldParam + '"]')
    assignCookieValueToFormInput(fieldParam, inputs)
  }

  /**
   * Do not edit.
   */
  function assignCookieValueToFormInput(fieldParam, inputs) {
    var field = getCookie(fieldParam),
      length = inputs.length

    if (field && length) {
      for (var i = 0; i < length; i++) {
        inputs[i].value = field
      }
    }
  }

  /**
   * Utility functions. Do not edit.
   */
  function getCookie(name) {
    var value = '; ' + document.cookie
    var parts = value.split('; ' + name + '=')
    if (parts.length == 2) return parts.pop().split(';').shift()
  }
  function setCookie(name, value, days) {
    var date = new Date()
    date.setTime(date.getTime() + days * 24 * 60 * 60 * 1000)
    var expires = '; expires=' + date.toGMTString()
    document.cookie = name + '=' + value + expires + ';path=/'
  }
  function getParam(p) {
    var match = RegExp('[?&]' + p + '=([^&]*)').exec(window.location.search)
    return match && decodeURIComponent(match[1].replace(/\+/g, ' '))
  }
}

/**
 * Example usage!
 * `window.onload` is necessary to ensure all form fields have loaded.
 */
window.onload = function () {
  assignTrackingParameterToCookie('gclid', 'hubspot')
  assignTrackingParameterToCookie('utm_source', 'gform')
  assignTrackingParameterToCookie('<YOUR_UTM>', '<YOUR_FORM_TYPE>')
}

Line 21 triggers an error in GTM. I changed it to function assignTrackingParameterToCookie(fieldParam, formType) { and everything else worked as expected. Thanks for this!