If you have correctly transferred the names of the free fields from the backend of the store in the store interface for the free field mapping, it may be that the free field names for the API differ. You can check this by adding the following to the store URL (with API at the end): "/articles/ID (ID = ID of the article in the store). There you check the JSON file for the correct name of the free fields. To access data, you have to use the API username and the API key.
Your technical contact in your company can help you with this.
You can find more information in our manual: