use DrupalCoreFormFormStateInterface;

function MODULENAME_form_alter(&$form,$form_state, $form_id) {
  if ($form_id == 'FORMID') {
    $form['actions']['submit']['#validate'][] = 'MODULENAME_form_validate';
  }
}

function MODULENAME_form_validate($form, FormStateInterface $form_state) {
  $form_state->setValue('hidden_id', 'Value for Hidden field');
}

/**
 * Implements hook_ENTITY_TYPE_presave().
 */

function webform_presave_webform_submission_presave(DrupalwebformEntityWebformSubmission $submission) {
  $submissionCreated = $submission->getCreatedTime();
  $submissionCreatedChnaged = format_date($submissionCreated, 'custom_date');
  $a = strptime($submissionCreatedChnaged, '%Y-%m-%d');
  $timestamp = mktime(0, 0, 0, $a['tm_mon']+1, $a['tm_mday'], $a['tm_year']+1900);
  $submission->setCreatedTime($timestamp);
}