I'm new to the 3 scale API management. I have configured a backend API, however this private API requires an API Key to authenticate. That being said, no where on the 3 scale backend configuration I can configure an API Key.
I'm aware that the APIcast requires a Secret Token to forward the request to the backend. From that point, my Private API requires an API Key.
I think you should use "Header Modification Policy" (YourProduct -> Integration -> Policies) to add or update your header with your API Key value.