ConfigSimpleExampleSettingsForm.php

<?php

namespace Drupal\config_simple_example\Form;

use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Configure example settings for this site.
 */
class ConfigSimpleExampleSettingsForm extends ConfigFormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'config_simple_example_settings';
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'config_simple_example.settings',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('config_simple_example.settings');
    $form['message'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Message'),
      '#default_value' => $config->get('message'),
    ];
    return parent::buildForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Retrieve the configuration.
    $this->config('config_simple_example.settings')
      ->set('message', $form_state->getValue('message'))
      ->save();
    parent::submitForm($form, $form_state);
  }

}