ClientID Метрики в форме Tilda
готовый код.

<script>
(function () {
  var COUNTER_ID = 12345678;
  var clientId = '';
  var attempts = 0;

  function fillFields() {
    if (!clientId) return;
    document.querySelectorAll('form').forEach(function (form) {
      var field = form.querySelector('input[name="metrika_client_id"]');
      if (!field) {
        field = document.createElement('input');
        field.type = 'hidden';
        field.name = 'metrika_client_id';
        form.appendChild(field);
      }
      field.value = clientId;
    });
  }

  function requestClientId() {
    attempts += 1;
    if (typeof window.ym === 'function') {
      window.ym(COUNTER_ID, 'getClientID', function (value) {
        clientId = value || '';
        fillFields();
      });
      return;
    }
    if (attempts < 20) window.setTimeout(requestClientId, 500);
  }

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