site_info.module

<?php

/**
 * @file
 * Contains site_info.module.
 */

use Drupal\node\Entity\Node;
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter(). Adds a new field for entering the API key to the Basic Site Settings page.
 * Retrieves API key system variable from config. If no value is set, displays default value 'No API Key yet'.
 * On form submit, direct to site_info_form_submit function to save the new API key value in config.
 */
function site_info_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'system_site_information_settings') {
    $config = \Drupal::service('config.factory')->getEditable('site_info.settings');
    $field_api_content = $config->get('apisitekey');
    $field_api_content = (!$field_api_content ? 'No API Key yet' : $field_api_content);
    $form['field_site_api_key'] = array (
                '#title' => t('Site API Key'),
                '#type' => 'textfield',
                '#default_value' => $field_api_content,
                '#description' => 'Please enter it',
                );

    $form['actions']['submit']['#submit'][] = 'site_info_form_submit';
  }
}


/**
 * Set the config for the siteapikey system variable from the value passed through $form_state.
 * Display the API key entered through the form to the user after submit.
 */
function site_info_form_submit($form, $form_state) {
  $config = \Drupal::service('config.factory')->getEditable('site_info.settings');
  $config->set('apisitekey', $form_state->getValue('field_site_api_key'))->save();
  drupal_set_message('The Site API Key has been saved with the value: ' . $config->get('apisitekey'));
}