UTM-метки в формах Tilda
готовый код.

<script>
(function () {
  var KEYS = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'];
  var STORE = 'tselno_utm';
  var data = {};

  try { data = JSON.parse(sessionStorage.getItem(STORE) || '{}'); } catch (error) {}

  var params = new URLSearchParams(window.location.search);
  KEYS.forEach(function (key) {
    if (params.get(key)) data[key] = params.get(key);
  });
  try { sessionStorage.setItem(STORE, JSON.stringify(data)); } catch (error) {}

  function fillForms() {
    document.querySelectorAll('form').forEach(function (form) {
      KEYS.forEach(function (key) {
        if (!data[key]) return;
        var field = form.querySelector('input[name="' + key + '"]');
        if (!field) {
          field = document.createElement('input');
          field.type = 'hidden';
          field.name = key;
          form.appendChild(field);
        }
        field.value = data[key];
      });
    });
  }

  fillForms();
  new MutationObserver(fillForms).observe(document.body, { childList: true, subtree: true });
})();
</script>