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'],
);
}