Issue #3271332 by RenatoG: Create a help text for the multiple field...

    if ($form_object instanceof EntityFormInterface) {

      $qty_classes_per_block = 10;

      // Get config object.

      $config = $this->configFactory->getEditable('block_class.settings');

      if (!empty($config->get('qty_classes_per_block'))) {

        $qty_classes_per_block = $config->get('qty_classes_per_block');

      }

      $url_settings_page = Url::fromRoute('block_class.settings')->toString();

      $field_group_class = $this->t('Customize the styling of this block by adding CSS classes. The maximum of classes per block is @qty_classes_per_block@ but if you need you can update this value in the <a href="@$url_settings_page@">settings page</a>', [

        '@qty_classes_per_block@' => $qty_classes_per_block,

        '@$url_settings_page@' => $url_settings_page,

      ]);

      $form['class'] = [

        '#type' => 'details',

        '#title' => $this->t('Class'),

        '#open' => TRUE,

        '#description' => $field_group_class,

      ];

      /** @var \Drupal\block\BlockInterface $block */

      // Default value for maxlength.

      $maxlength_block_class_field = 255;

      // Get config object.

      $config = \Drupal::config('block_class.settings');

      // Get the field type if exists.

      if (!empty($config->get('field_type')) && $config->get('field_type') != 'multiple_textfields') {

        $field_type = $config->get('field_type');

      if ($config->get('field_type') == 'textfield' || $config->get('field_type') == 'textarea') {

        // Remove the help text in the field group because the field will have

        // their help text.

        unset($form['class']['#description']);

        $form['class']['third_party_settings']['block_class']['classes'] = [

          '#type' => $field_type,

          '#title' => $this->t('CSS class(es)'),

        // Explode by spaces to get all classes in an array.

        $classes = explode(' ', $classes);

        $qty_classes_per_block = 10;

        if (!empty($config->get('qty_classes_per_block'))) {

          $qty_classes_per_block = $config->get('qty_classes_per_block');

        }

        // Run a for to add 10 multiple fields.

        for ($index = 0; $index <= ($qty_classes_per_block - 1); $index++) {