$dir = "/home/nolatron/public_html/domain.com/forums";
require($dir . '/src/XF.php');
XF::start($dir);
$forumId = '20'; //Forum ID of where to post thread
$userId = '2'; //User ID # of account to post thread as
$title = "CONTENTS OF SUBJECT";
$message = 'CONTENTS OF POST';
$forum = \XF::em()->find('XF:Forum', $forumId);
$user = \XF::em()->find('XF:User', $userId);
\XF::asVisitor($user, function() use ($forum, $title, $message)
{
$creator = \XF::service('XF:Thread\Creator', $forum);
$creator->setContent($title, $message);
$creator->setPrefix('1'); //Prefix ID # to set new thread as.
$creator->setIsAutomated();
$creator->save();
});