Neos\Media\Browser\Controller\AssetController::createTagAction PHP Method

    public function createTagAction($label)
    {
        $existingTag = $this->tagRepository->findOneByLabel($label);
        if ($existingTag !== null) {
            if (($assetCollection = $this->browserState->get('activeAssetCollection')) !== null && $assetCollection->addTag($existingTag)) {
                $this->assetCollectionRepository->update($assetCollection);
                $this->addFlashMessage('tagAlreadyExistsAndAddedToCollection', '', Message::SEVERITY_OK, [htmlspecialchars($label)]);
            }
        } else {
            $tag = new Tag($label);
            $this->tagRepository->add($tag);
            if (($assetCollection = $this->browserState->get('activeAssetCollection')) !== null && $assetCollection->addTag($tag)) {
                $this->assetCollectionRepository->update($assetCollection);
            }
            $this->addFlashMessage('tagHasBeenCreated', '', Message::SEVERITY_OK, [htmlspecialchars($label)]);
        }
        $this->redirect('index');
    }