class TreeConfigTargetForm

class TreeConfigTargetForm extends ConfigFormBase {
    
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'form_test.object',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_tree_config_target_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['vegetables'] = [
            '#type' => 'details',
            '#open' => TRUE,
            '#tree' => TRUE,
            '#input' => TRUE,
            '#title' => t('Vegetable preferences'),
        ];
        $form['vegetables']['favorite'] = [
            '#type' => 'textfield',
            '#title' => t('Favorite'),
            '#default_value' => 'Potato',
            '#config_target' => 'form_test.object:favorite_vegetable',
        ];
        $form['vegetables']['nemesis'] = [
            '#type' => 'textfield',
            '#title' => t('Nemesis'),
            '#config_target' => 'form_test.object:nemesis_vegetable',
        ];
        // Since form_test.object contains translatable values, it must specify a
        // language.
        $form['langcode'] = [
            '#type' => 'value',
            '#value' => 'en',
            '#config_target' => 'form_test.object:langcode',
        ];
        $form['test1'] = [
            '#type' => 'select',
            '#title' => $this->t('Test 1'),
            '#options' => [
                'option1' => $this->t('Option 1'),
                'option2' => $this->t('Option 2'),
            ],
            '#ajax' => [
                'callback' => '::updateOptions',
                'wrapper' => 'edit-test1-wrapper',
            ],
            '#prefix' => '<div id="edit-test1-wrapper">',
            '#suffix' => '</div>',
        ];
        return parent::buildForm($form, $form_state);
    }
    
    /**
     * Updates the options of a select list.
     *
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     *
     * @return array
     *   The updated form element.
     */
    public function updateOptions(array $form, FormStateInterface $form_state) {
        $form['test1']['#options']['option1'] = $this->t('Option 1!!!');
        $form['test1']['#options'] += [
            'option3' => $this->t('Option 3'),
            'option4' => $this->t('Option 4'),
        ];
        return $form['test1'];
    }

}