From what I know DF will use the user rights of the user that has just registered even if you were to supply a different API key(bug???) so you need to make sure that your user has rights to do POST to your AWS service.
This can be very confusing to new people because they test things in API docs and don't realize that all API calls from API docs are being done from an admin account that basically has full access. When you test it from your application you will use an API key that has restrictions. The security is difficult to understand and I have been using DF for over a year.