public static String getAuthToken(){
        def http = new HTTPBuilder(url + "some/url")   
        http.request (POST) { multipartRequest ->
            MultipartEntityBuilder multipartRequestEntity = new MultipartEntityBuilder()
            String key = 
            multipartRequestEntity.addPart('Username', new StringBody(user))
            multipartRequestEntity.addPart('Password', new StringBody(password))

            multipartRequest.entity = multipartRequestEntity.build()

            response.success = { resp, data ->
                return data['token']
            }
            response.failure = { resp, data ->
                return data
            }
        }
    }

U can perform form filling by using MultipartRequestEntityBuilder