Add Drupal configuration form programmatically

drupalbook.info.yml

name: DrupalBook
description: Custom module for learning Drupal
type: module
core: 8.x
core_version_requirement: ^8 || ^9
package: DrupalBook

drupalbook.routing.yml

drupalbook.settings:
  path: '/admin/structure/drupalbook/settings'
  defaults:
    _form: '\Drupal\drupalbook\Form\DrupalbookSettingsForm'
    _title: 'DrupalBook Settings form'
  requirements:
    _permission: 'administer site configuration'

DrupalbookSettingsForm.php

<?php
// /modules/custom/drupalbook/src/Form/DrupalbookSettingsForm.php
namespace Drupal\drupalbook\Form;
 
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
 
/**
 * Configure example settings for this site.
 */
class DrupalbookSettingsForm extends ConfigFormBase {
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'drupalbook_admin_settings';
  }
 
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'drupalbook.settings',
    ];
  }
 
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('drupalbook.settings');
 
    $form['drupalbook_api_key'] = array(
      '#type' => 'textfield',
      '#title' => $this->t('API Key'),
      '#default_value' => $config->get('drupalbook_api_key'),
    );
 
    $form['drupalbook_api_client_id'] = array(
      '#type' => 'textfield',
      '#title' => $this->t('API Client ID'),
      '#default_value' => $config->get('drupalbook_api_client_id'),
    );
 
    return parent::buildForm($form, $form_state);
  }
 
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Retrieve the configuration
    $this->configFactory->getEditable('drupalbook.settings')
      // Set the submitted configuration setting
      ->set('drupalbook_api_key', $form_state->getValue('drupalbook_api_key'))
      // You can set multiple configurations at once by making
      // multiple calls to set()
      ->set('drupalbook_api_client_id', $form_state->getValue('drupalbook_api_client_id'))
      ->save();
 
    parent::submitForm($form, $form_state);
  }
}