sak.module

<?php


/**
 * @file
 *
 *  Convert given basic page node to json using api key.
 */

/**
 * Implements hook_form_FORM_ID_alter().
 */
function sak_form_system_site_information_settings_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  // Get the API Key configuration
  $config = \Drupal::config('sak.settings');

  // Check whether the API value is set by the user or empty.
  if(empty($config->get('sakkey')) ) {
    $site_api =  $config->get('sakkey');
  }

  // Create Site API Key Textfield
  $form['site_information']['site_api_key'] = [
    '#type' => 'textfield',
    '#title' => t('Site API Key'),
    '#placeholder' =>t('Add Site API Key here'),
    '#required' => TRUE,
    '#default_value' => $site_api,
  ];


  // Change the text of "Save configuration" button to "Update Configuration".
  if ( !empty($config->get('sakkey')) && $site_api != '' ) {
    $form['actions']['submit']['#value'] = t('Update Configuration');
  }

  // Add custom submit handler
  $form['#submit'][] = 'sak_system_site_information_settings_submit_handler';

}


/**
 * Custom Submit handler for FormID system_site_information_settings.
 */
function sak_system_site_information_settings_submit_handler(&$form,\Drupal\Core\Form\FormStateInterface $form_state ) {

  // Get the value from Form State
  $site_api_key = $form_state->getValue('site_api_key');

  // Set the sakkey value using configuration API.
  \Drupal::configFactory()->getEditable('sak.settings')->set('sakkey', $site_api_key)->save();;


  // Show a warning message if the user enters and saves an empty API Key value.
  if (empty($site_api_key) || $site_api_key === 'No API Key yet' ) {
    drupal_set_message(t('You have not set your Site\'s API Key. Please set the API key below and Save Configuration.'), 'warning');
  }

  // Inform the user that the Site API Key has been saved successfully.
  else {
    drupal_set_message(t('You have successfully set your Site\'s API Key to  @site_api_key .', array('@site_api_key' => $site_api_key)));
  }

}