code: public function buildForm(array $form, FormStateInterface $form_state) {
  
      $form = parent::buildForm($form, $form_state);
  
      $form['google_analytics']['ga_account'] = [
        '#description' => t('This ID is unique to each site you want to track separately, and is in the form of UA-xxxxxxx-yy. To get a Web Property ID, <a href=":analytics" target="_blank">register your site with Google Analytics</a>, or if you already have registered your site, go to your Google Analytics Settings page to see the ID next to every site profile. <a href=":webpropertyid"  target="_blank">Find more information in the documentation</a>.', [
          ':analytics' => 'http://www.google.com/analytics/',
          ':webpropertyid' => Url::fromUri('https://developers.google.com/analytics/resources/concepts/gaConceptsAccounts', ['fragment' => 'webProperty'])
            ->toString(),
        ]),
        '#maxlength' => 20,
        '#placeholder' => 'UA-',
        '#size' => 15,
        '#title' => t('Web Property ID'),
        '#type' => 'textfield',
      ];
  
      return $form;
    }
docs: {@inheritdoc}