code: public function SearchForm()
      {
          // If we have setup objects to search
          if (count(Searchable::config()->objects)) {
              $searchText =  "";
  
              if ($this->owner->request && $this->owner->request->getVar('Search')) {
                  $searchText = $this->owner->request->getVar('Search');
              }
  
              $fields = FieldList::create(
                  TextField::create('Search', false, $searchText)
                      ->setAttribute("placeholder", _t('Searchable.Search', 'Search'))
              );
  
              $actions = FieldList::create(
                  FormAction::create('results', _t('Searchable.Go', 'Go'))
              );
  
              $template_class = Searchable::config()->template_class;
              $results_page = Injector::inst()->create($template_class);
  
              $form = Form::create(
                  $this->owner,
                  'SearchForm',
                  $fields,
                  $actions
              )->setFormMethod('get')
              ->setFormAction($results_page->Link())
              ->setTemplate('ilateral\SilverStripe\Searchable\Includes\SearchForm')
              ->disableSecurityToken();
  
              $this->owner->extend("updateSearchForm", $form);
  
              return $form;
          }
      }
docs: Add a site search form to all controllers that links to the
  results controller.
  
  @return Form