Dependencies
$checkboxes = $k->checkboxes('foo')
->setTitle($this->t('Foo'))
->setDescription($this->t('Foo description.'))
->appendOption('bar', $this->t('Bar')),
->appendOption('baz', $this->t('Baz')),
->appendOption('qux', $this->t('Qux')),
->setDefaultValue(['baz']);