Форум

\Bitrix\Main\Loader::IncludeModule('tasks');
\Bitrix\Main\Loader::IncludeModule('forum');

/**
 * @var int ID задачи
 */
$taskId = 1;

/**
 * @var int ID форума
 */
$forumId = \Bitrix\Tasks\Integration\Forum\Task\Comment::getForumId();


$messages = \Bitrix\Forum\MessageTable::getList([
   'select' => ['ID','POST_MESSAGE','AUTHOR_ID'],
   'filter' => [
      'REAL_TOPIC.FORUM_ID' => $forumId,
      'REAL_TOPIC.XML_ID'   => 'TASK_'.$taskId,
      '!=PARAM1' => 'TK',
   ],
   'runtime' => [
      new \Bitrix\Main\Entity\ReferenceField(
         'REAL_TOPIC',
         '\Bitrix\Forum\TopicTable',
         array('=this.TOPIC_ID' => 'ref.ID')
      )

   ],
]);

foreach ($messages as $message)
{
   var_dump([
      'ID'           => $message['ID'],
      'POST_MESSAGE' => $message['POST_MESSAGE'],
      'AUTHOR_ID'    => $message['AUTHOR_ID'],
   ]);
}