APIs published on the WSO2 API Cloud are secured using OAuth2.0 by default. Any client application invoking a secure published API needs to have a valid subscription to the particular API and present a valid OAuth2.0 Access Token to the API Gateway.
The HTTP Authorization header is the most common method of providing authentication information for REST APIs and it is used in API Cloud as well. The application needs to have the access token in the Authorization header to authenticate the API that is being accessed. But, there can be reasons such as organizational policies, legacy backends expecting to use the authorization header for other purposes and legacy client applications which will force you to use some other header to pass the bearer token to the API gateway. https://goo.gl/zEkH1c