json_node.module

<?php

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

function json_node_form_system_site_information_settings_alter (array &$form, FormStateInterface $form_state, $form_id) {
	
	$dvalue = 'No API Key yet';
	$api = \Drupal::configFactory()->getEditable('system.site')->get('siteapikey');
	if($api){
		$dvalue = $api;
		$form['actions']['submit']['#value'] = t('Update Configuration');
	}
	$form['site_api_key'] = [
      '#type' => 'textfield',
      '#title' => t('Site API Key'),
      '#default_value' => $dvalue ,
	  '#weight' => -10
    ];
	$form['#submit'][] = 'json_node_extra';
}

function json_node_extra ($form, FormStateInterface $form_state) {
	 \Drupal::configFactory()->getEditable('system.site')
	->set('siteapikey', $form_state->getValue('site_api_key'))
    ->save();
	drupal_set_message('Site API Key has been saved as -> '. $form_state->getValue('site_api_key'));
}