Implements hook_field_widget_settings_form().
function field_test_field_widget_settings_form($field, $instance) {
$widget = $instance['widget'];
$settings = $widget['settings'];
$form['test_widget_setting'] = array(
'#type' => 'textfield',
'#title' => t('Field test field widget setting'),
'#default_value' => $settings['test_widget_setting'],
'#required' => FALSE,
'#description' => t('A dummy form element to simulate field widget setting.'),
);
return $form;
}