Change form before submitting it

There are 2 entities: Client and Apps.

The association between client and app is done in entity Client as follows:

class Clients {

    /**
    * App id
    *
    * @ManyToOne(targetEntity="Apps", inversedBy="clients", fetch="EXTRA_LAZY")
    * @JoinColumn(name="app_id", referencedColumnName="id")
    * @Assert\NotBlank(message="Código da aplicação inválido.")
    */
    private $appId;

    //...
}

The problem is that I do not want to expose any id in the form, so each app besides id, has the field reference and it is this value that is sent by the form, see below the build of the form:

class ClientType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('reference', TextType::class, [
                'mapped' => false
            ])
            ->add('title', TextType::class)
            ->add('firstname', TextType::class)
            ->add('lastname', TextType::class)
           ->add('phone', TextType::class)
           ->add('email', EmailType::class)
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Clients',
            'csrf_protection' => false,
        ));
    }
}

Notice that the field reference is false, that is, it does not exist in my entity but the field appId.

Ao save my form, the field appId goes with the value null and the following validation error occurs (the message was set via annotation in the code above):

Invalid application code.

In the controller, I have the following code:

//...
$data = json_decode($request->getContent(), true);
$client = new Clients();

$form = $this->createForm(ClientType::class, $client);
$form->submit($data);

if ($form->isValid()) {
   //...
}
//...

Before submitting the form, I need to fill in the field appId with a valid id according to the reference sent through the field reference.