Add a class to an anchor?

gistfile1.txt

function my_module_entity_view(&$build, EntityInterface $entity, EntityViewDisplayInterface $display) {
  if ($entity instanceof FieldableEntityInterface && $entity->hasField('field_name')) {
    foreach (Element::children($build['field_name']) as $item) {
      $build['field_name'][$item]['#attributes'] = ['class' => 'my-class'];
    }
  }
}