Add content to the API response body

We have external webpage where in we call the Zendesk api (/api/v2/apps/notify.json) which in turns triggers a custom app deployed in zendesk. The custom app performs few validation and needs to send back custom field values from the ticket to the webpage.how can we send the custom messages back to the webpage as response.Note: Currenlty this api when called returns only the status ie(200) and the body of the response is empty.i need to append some data to the body of the response