#include <IE.au3>
$sUrl = 'http://autoit-script.ru/index.php?action=forum'
$sSearch = 'HelpMe'
$oIE = _IECreate($sUrl)
$oForm = _IEFormGetCollection($oIE, 1)
$oSearch = _IEFormElementGetObjByName($oForm, 'search')
$oSubmit = _IEFormElementGetObjByName($oForm, 'submit')
_IEFormElementSetValue($oSearch, $sSearch)
_IEAction($oSubmit, 'click')