Thank you for clarifying. If the API is published via the portal you will need to ensure that you have chosen OAuth from the policy templates. Once the API is synced to your Gateway you will need to use the OAuth endpoints on the external Gateway to generate an access token.
The OAuth endpoints utilized will be on the same Gateway as the published API. To consume the API you must first follow the steps to obtain the token
OAuth Request Scenarios - CA API Management OAuth Toolkit - 4.3 - CA Technologies Documentation
Once you have the token it can then be passed to the API as either part of the form POST body, authorization header or optionally, though not recommended, via the query parameter.
If I have misunderstood please do let me know.
Regards,
Joe