Issue #3270661 by RenatoG: Add a visual helptext in the description of class...

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

      }

      $image_path = '/' . drupal_get_path('module', 'block_class') . '/images/';

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

        '#type' => $field_type,

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

        '#description' => $this->t('Customize the styling of this block by adding CSS classes. Separate multiple classes by spaces. The maxlength configured is @maxlength_block_class_field@. If necessary you can update it <a href="/admin/config/content/block-class/settings">here</a>', [

        '#description' => $this->t('Customize the styling of this block by adding CSS classes. Separate multiple classes by spaces. The maxlength configured is @maxlength_block_class_field@. If necessary you can update it <a href="/admin/config/content/block-class/settings">here</a>. This class will appear in the first level of block. <a href="@image_path@/example-1.png">Click here</a> to see a example', [

          '@maxlength_block_class_field@' => $maxlength_block_class_field,

          '@image_path@' => $image_path,

        ]),

        '#default_value' => $block->getThirdPartySetting('block_class', 'classes'),

        '#maxlength' => $maxlength_block_class_field,