MyFormType.php

<?php

namespace MyBundle\Form\Type;

use Symfony\Component\DependencyInjection\ContainerAwareInterface;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Validator\Constraints as Assert;

class MyFormType extends AbstractType implements ContainerAwareInterface
{
    use \Symfony\Component\DependencyInjection\ContainerAwareTrait;

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $translator = $this->container->get('translator');
        
        $builder->add('my-field', 'text', [
            'constraints' => [
                new Assert\NotBlank([
                    'message' => $translator->trans('%field% should not be blank.', ['%field%' => $translator->trans('MyFieldName')]),
                ]),
            ],
        ]);
    }

    public function getName()
    {
        return 'my_form';
    }
}