You should use a menulist control, and a datasource, inheriting from jFormsDaoDatasource or jFormsDynamicDatasource, and having a getData() method returning an empty list.

The widget accepts a specific attribute, 'attr-autocomplete', an array which should contains at least an item 'source' indicating the url of the search engine. The array may contains other attributes for the input element used to type the search term (class, style..).

example of use:

In the form file:

    <menulist ref="mylist"> <label>test</label>
    <datasource class="mymodule~mydatasource"/>
    </menulist>

The datasource:

class mydatasource extends jFormsDaoDatasource {
   function __construct($formId) {
      parent::__construct ("mymodule~myDao" , "findAll" , "label", 'id');
   }
   public function getData($form) { return array(); }
 }

In a template:

{form $form, $submitAction, $submitParam, 'html', array('plugins'=>array(
     'mylist'=>'autocompleteajax_html'))}

{formcontrols}
   ... {ifctrl 'mylist'}{ctrl_control '', array(
         'attr-autocomplete'=>array('style'=>'width:40em;',
             'source'=>$searchUrl))}
       {else}{ctrl_control}{/ifctrl}
{/formcontrols}