Form submit callback for the preview button.
public function preview(array $form, FormStateInterface $form_state) {
$task_item = $this->entity;
$job_item = $task_item
->getJobItem();
$job_item
->addTranslatedData($this
->prepareData($task_item
->getData()), [], TMGMT_DATA_ITEM_STATE_PRELIMINARY);
/** @var \Drupal\Core\Url $url */
$url = $job_item
->getSourcePlugin()
->getPreviewUrl($job_item);
$form_state
->setRedirectUrl($url);
}