/**
 * Implements hook_field_widget_third_party_settings_form().
 */
function {{ machine_name }}_field_widget_third_party_settings_form(\Drupal\Core\Field\WidgetInterface $plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $form_mode, array $form, \Drupal\Core\Form\FormStateInterface $form_state) {
  $element = [];
  // Add a 'my_setting' checkbox to the settings form for 'foo_widget' field
  // widgets.
  if ($plugin->getPluginId() == 'foo_widget') {
    $element['my_setting'] = [
      '#type' => 'checkbox',
      '#title' => t('My setting'),
      '#default_value' => $plugin->getThirdPartySetting('my_module', 'my_setting'),
    ];
  }
  return $element;
}