Pimcore\Model\Element\Tag::setParentId PHP Method

    public function setParentId($parentId)
    {
        $this->parentId = $parentId;
        $this->parent = null;
        $this->correctPath();
        return $this;
    }

 public function addAction()
 {
     $tag = new Pimcore\Model\Element\Tag();
     $tag->setName(strip_tags($this->getParam('text')));
     $tag->setParentId(intval($this->getParam('parentId')));
     $tag->save();
     $this->_helper->json(['success' => true, 'id' => $tag->getId()]);
 }