I don't know if there's anything particular to subforms, and also I don't exactly remember how this works, but what I've got is setting the save URL with
$saveOrderingUrl = 'index.php?option=com_mobilemanege&task=tests.saveOrderAjax&tmpl=component&' . Session::getFormToken() . '=1';
and setting up the list with
HTMLHelper::_('draggablelist.draggable');
and presumably you've already got the body of the table properly class-ified with
class="js-draggable" data-url="<?php echo $saveOrderingUrl; ?>" data-direction="<?php echo strtolower($listDirn); ?>" data-nested="true"
?