code: public function buildForm(array $form, FormStateInterface $form_state) {
  
      $form = parent::buildForm($form, $form_state);
  
      $form['thunder_liveblog']['description'] = [
        '#type' => 'item',
        '#markup' => $this->t('Register a new account at <a href=":pusher_url" target="_blank">:pusher_url</a>, create a new app and note down your keys and cluster. You can provide them right here or at a later stage on the liveblog settings form.',
          [':pusher_url' => 'http://pusher.com']),
      ];
  
      $form['thunder_liveblog']['pusher_app_id'] = [
        '#type' => 'textfield',
        '#title' => t('App ID'),
      ];
  
      $form['thunder_liveblog']['pusher_key'] = [
        '#type' => 'textfield',
        '#title' => t('Key'),
      ];
  
      $form['thunder_liveblog']['pusher_secret'] = [
        '#type' => 'textfield',
        '#title' => t('Secret'),
      ];
  
      $form['thunder_liveblog']['pusher_cluster'] = [
        '#type' => 'textfield',
        '#title' => t('Cluster'),
        '#description' => t('The cluster name to connect to. Leave empty for the default cluster: mt1 (US east coast)'),
      ];
  
      return $form;
    }
docs: {@inheritdoc}