$form = \Atk4\Ui\Form::addTo($app);
$recipient = $form->addControl(
    'recipient',
    [\Atk4\Ui\Form\Control\Dropdown::class,
        'isMultiple' => true,
        'dropdownOptions' => ['allowAdditions' => true, 'forceSelection' => false],
    ],
    ['default' => 'Username <user@emaildomain.de>']
);
$form->onSubmit(function () use ($form) {
    echo '<pre>';
    echo htmlspecialchars(
        print_r($form->model->get(), true)
    );
});