use Symfony\Component\Form\Extension\Core\Type\WeekType;

$builder->add('startDateTime', WeekType::class, [
    // use this if you store week numbers as strings ('2011-W17')
    'input' => 'string',
    // use this if you store week numbers as arrays (e.g. [2011, 17])
    'input' => 'array',

    // renders two <select> to select the year and week number
    'widget' => 'choice',
    // renders two <input type="text"> to write the year and week number
    'widget' => 'text',
    // renders a <input type="week"> which is properly rendered by most browsers
    'widget' => 'single_text',
]);