code: public function execute()
      {
          $result = $this->resultJson->create();
  
          try {
              $dictionaryId = $this->getRequest()->getParam('dictionary_id');
              $value = $this->getRequest()->getParam('item_value');
              $key = $this->getRequest()->getParam('item_key');
              $this->api->upsertDictionaryItem($dictionaryId, $key, $value);
  
              return $result->setData(['status' => true]);
          } catch (\Exception $e) {
              return $result->setData([
                  'status'    => false,
                  'msg'       => $e->getMessage()
              ]);
          }
      }
docs: Create dictionary item
  
  @return $this|\Magento\Framework\App\ResponseInterface|\Magento\Framework\Controller\ResultInterface