I'm wondering that too.
I want to search the product InternalItemID (productcode)

SS 2.3.4
mysite > page.php

   /**
    * Site search form
    */
   function SearchForm() {
      $searchText = isset($_REQUEST['Search']) ? $_REQUEST['Search'] : 'Search';
      $fields = new FieldSet(
       new TextField("Search", "", $searchText)
       );
      $actions = new FieldSet(
       new FormAction('results', 'Search')
       );

       return new SearchForm($this, "SearchForm", $fields, $actions);
   }
   
   /**
    * Process and render search results
    */
   function results($data, $form){
       $data = array(
       'Results' => $form->getResults(),
       'Query' => $form->getSearchQuery(),
       'Title' => 'Search Results'
       );

       return $this->customise($data)->renderWith(array('Page_results', 'Page'));
   }