class SearchProductTypeExtension extends AbstractTypeExtension
{
    public $app;

    public function __construct($app)
    {
        $this->app = $app;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $app = $this->app;
        if (!$app->isGranted('ROLE_ADMIN')) {
            return;
        }

        // フォームを追加するためにイベントリスナーを追加
       $builder->addEventListener(FormEvents::POST_SET_DATA, function (FormEvent $event) use ($app) {

            $choices = array();

            // 選択肢追加
            $form = $event->getForm();
            $form->add('plg_test_id', 'choice', array(
                'mapped' => true,
                'required' => false,
                'choices' => $choices,
                'empty_value' => '選択してください',
            ));
       });
    }

    public function getExtendedType()
    {
        return 'admin_search_product';
    }
}

# 追加商品検索
admin.order.edit.search.product.search:
    - [onAdminProductIndexSearch, NORMAL]

    public function onAdminProductIndexSearch(EventArgs $event)
    {
        $app = $this->app;
        if (!$app->isGranted('ROLE_ADMIN')) {
            return;
        }

        $searchData = $event->getArgument('searchData');
        if (empty($searchData)) {
            return;
        }
var_dump($searchData);  // ★plg_test_id が設定されていない

        $request = $event->getRequest();
        if ($request) {
            $searchData['plg_test_id'] = $request->get('plg_test_id');
        }
var_dump($request);  // ★plg_test_id が設定されていない
die('OK');

        ...

    }