Invoke external REST service form IBM BAW19.x

Question

I need to invoke external webservice from IBM BAW 19.x. All documentation I referred to points to having a swagger file for the REST service, then create External Service using the swagger which will create all service artifacts (methods, parameters etc.) and then create a service flow which will use this service. My problem is I do not have a swagger file for this external REST service. Is there any other way to do this? Thanks in advance