I have a form with a contact list. I want the field "first name" appear with the selected contact value after submit. My problem is that the field appear but I cant set the good data, the field always remains empty.
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('contacts', ChoiceType::class, [
            'label'       => 'Contact',
            'placeholder' => 'Choose a contact',
            'choices'     => $this->getContacts(),
            'mapped'      => false,
        ])
        ->setMethod('POST')
    ;
    $builder->get('contacts')->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) {
        $contactId     = $event->getData();
        $parentForm    = $event->getForm()->getParent();
        $contactEntity = $exampleEm->getrepository(Contact::class)->find($contactId);
        $firstName     = $contactEntity->getFirstName();
        // where can I set the 'contactFirstname' data ?
        $parentForm
            ->add('contactFirstname', TextType::class, [
                'label' => 'First name',
            ]);
    })
    ;
}
How to enter the right data so that the field appears pre-filled?
Edit : I found a method, but it's not terrible:
$parentForm
            ->add('contactFirstname', TextType::class, [
                'label'         => 'First name',
                'empty_data'    => $firstName,
        ]);
('data' => $firstNamedont work for me.)   
$parentForm->get('contactFirstname')->setData($firstName); doesn't work either
 
     
     
     
    