/**
 * Implements hook_field_widget_complete_WIDGET_TYPE_form_alter().
 */
function {{ machine_name }}_field_widget_complete_WIDGET_TYPE_form_alter(&$field_widget_complete_form, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
  $field_widget_complete_form['#attributes']['class'][] = 'my-class';
}
