/**
 * Implements hook_field_widget_single_element_form_alter().
 */
function {{ machine_name }}_field_widget_single_element_form_alter(array &$element, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
  // Add a css class to widget form elements for all fields of type my_type.
  $field_definition = $context['items']->getFieldDefinition();
  if ($field_definition->getType() == 'my_type') {
    // Be sure not to overwrite existing attributes.
    $element['#attributes']['class'][] = 'my-class';
  }
}