How to add a new resource to a folder

    $parentFolder = $modx->getObject('modResource', 100);

    $newFolder = $modx->newObject('modResource');
    $newFolder->set('pagetitle', $year);
    $newFolder->set('alias', "year-".$year);
    $newFolder->set('isfolder', 1);
    $newFolder->set('Parent', $parentFolder);

    $newFolder->save();

$parentFolder = 100;
$newFolder->set('parent', $parentFolder);