Provide the default form for setting options.
Overrides ArgumentDefaultPluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
  parent::buildOptionsForm($form, $form_state);
  $form['argument'] = array(
    '#type' => 'textfield',
    '#title' => t('Fixed value'),
    '#default_value' => $this->options['argument'],
  );
}