site_information.module

<?php
/**
* @file
* Add a field to a site information form.
*/

/**
 * Implement hook_form_FORM_ID_alter().
 */
function site_information_form_system_site_information_settings_alter(&$form, $form_state, $form_id) {
    
    $form['site_information']['site_information_apikey'] = array(
        '#type' => 'textfield',
        '#title' => t('Site API Key'),
        '#default_value' => \Drupal::config('system.site')->get('site_information_apikey'),
    );
    $form['actions']['submit']['#value'] = t('Update Configuration');
    $form['#submit'][] = 'system_site_information_set_value';
        

}

/**
 *  @callback function
 */
function system_site_information_set_value($form, &$form_state) {

 \Drupal::configFactory()->getEditable('system.site')->set('site_information_apikey', $form_state->getValue('site_information_apikey'))->save();

}