public class ParamController : ApiController
    {
        [HttpPost]
        [Route("doLoginExt")]
        public HttpResponseMessage doLoginExt([FromUri]string istituto, string password)
        {
            
            WebRequest request = WebRequest.Create("https://....servizio..../pagina.php?parametri=aaa,bbb,ccc&metodoRest=doLogin");
            
            request.Credentials = CredentialCache.DefaultCredentials;
          
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
           
            Stream dataStream = response.GetResponseStream();
           
            StreamReader reader = new StreamReader(dataStream);
          
            string responseFromServer = reader.ReadToEnd();

            common_login sessione = JsonConvert.DeserializeObject<common_login>(responseFromServer);
            
            reader.Close();
            dataStream.Close();
            response.Close();

            HttpResponseMessage resp = new HttpResponseMessage();

            JsonMediaTypeFormatter formatoJson = new JsonMediaTypeFormatter();

            resp.Content = new ObjectContent<common_login>(sessione, formatoJson);

            return resp;            
        }
    }

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;