How to prefill form values with Symfony 4 form listeners?

I would like to prefill form values from a form event listener.

I tried this, but nothing happens:

public function buildForm(FormBuilderInterface $builder, array $options)
{
    /* ...*/

    $builder->addEventListener(
        FormEvents::PRE_SET_DATA,
        [$this, 'onPreSetData']
    );
}

public function onPreSetData(FormEvent $event)
{
   $event->getForm()->get('myfield')->setData('my value');
}

Thanks all!

Alexander