Flow API custom headers

I've set up a flow that interacts with other services using the REST connector. From this flow I've created a Flow API by multi selecting the entire flow, right clicking it and selecting the API settings option. 

Inside the flow I extract a response header from one of the REST requests respones I receive in the flow and I want that value to show up as an output header of the entire Flow API response whenever this service gets called from the outside. Right now I only can see the default response headers. 

I've read and search the documentation pertinent to these elements but I haven't found out how I can create and parse custom response headers for the Flow API and if anyone knows how to do that, your help will be immensely appreciated 😬