How to use Custom Response Schema in version 5

Has anyone a sample of Custom Response Schema in version 5? I have used in version 4 with the wrapper option IsCustomSchema=true which is removed in version 5. I tried with a class inheriting from IApiResponse but cannot get it to work. I wonder if the test for IApiResponse in HandleSuccessfulRequestAsync is correct or should have a test for there is an interface of type IApiResponse instead?