I found fragment of code at "/packages/acs-subsite/lib/user-info.tcl"
that can easily answer this endeless question.
ad_form -name user_info -cancel_url $return_url -action $action_url -mode $form_mode -form {
{user_id:integer(hidden),optional}
{return_url:text(hidden),optional}
{message:text(hidden),optional}
}
Then,
I ammended, reloaded the change. But surprisely it seems it doesn't work out.
"/packages/acs-subsite/lib/user-new.tcl"
ad_form -name register -export {next_url user_id return_url} -form [auth::get_registration_form_elements]
{url:text(hidden),optional}
{screen_name:url(hidden),optional}
does anyone know a way around?