Answered Plugin for resources with specific template

<?php
/* keep this from running in the Manager */
if ($modx->context->get('key') == 'mgr') {
    return;
}
 
/* save the new alias based on pagename */
$resource->set('alias', '');
$resource->save();

<?php
/* keep this from running in the Manager */
if ($modx->context->get('key') == 'mgr') {
    return;
}
 
/* (optional) Don't execute for existing resources --
   Remove the following line to let users modify
   existing parents in NewsPublisher
 
if (!($mode == modSystemEvent::MODE_NEW)) return;  */

/* get parent ID from TV */
$parentId = $resource->getTVValue('index_category');

/* save the parent id in the resource's parent field */
$resource->set('parent', $parentId);
$resource->save();

if ($mode == modSystemEvent::MODE_UPD) {
}

if ($mode == modSystemEvent::MODE_NEW) {
}

$modx->invokeEvent('OnBeforeDocFormSave',array(
    'mode' => modSystemEvent::MODE_NEW,
    ...

class modSystemEvent {
    /**
     * @var string For new creations of objects in model events
     */
    const MODE_NEW = 'new';
    /**
     * @var string For updating objects in model events
     */
    const MODE_UPD = 'upd';