Provide the default form for setting options.
Overrides ArgumentDefaultPluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
// Using range(1, 10) will create an array keyed 0-9, which allows arg() to
// properly function since it is also zero-based.
$form['index'] = array(
'#type' => 'select',
'#title' => t('Path component'),
'#default_value' => $this->options['index'],
'#options' => range(1, 10),
'#description' => t('The numbering starts from 1, e.g. on the page admin/structure/types, the 3rd path component is "types".'),
);
$form['use_alias'] = array(
'#type' => 'checkbox',
'#title' => t('Use path alias'),
'#default_value' => $this->options['use_alias'],
'#description' => t('Use path alias instead of internal path.'),
);
}