ControllerAPI.cs

//ESSA É A CONTROLLER DA API QUE RECEBE OS DADOS (ASPNET CORE)

[Produces("application/json")]
    [Route("api/cupons/[action]")]
    public class CuponsController : Controller
    {

        private readonly CupomDAO _cupomDao;
        private readonly ClienteDAO _clienteDao;

        public CuponsController(IConfiguration configuration)
        {
            _cupomDao = new CupomDAO(configuration);
            _clienteDao = new ClienteDAO(configuration);
        }

        // POST: api/Cupons
        [HttpPost]
        public string VerificaCupom(string idParceiro, string codigo)
        {
            return codigo;
            //return _cupomDao.CupomClienteParceiro(idParceiro, codigo);
        }
    }

RestApp.cs

//ESSE É O HTTPCLIENT DO APP QUE ENVIA OS DADOS

using (var client = new HttpClient())
                {
                    client.BaseAddress = new Uri(Settings.RestAPISettings);
                    // Add string parameters
                    var content = new FormUrlEncodedContent(new[] {
                        new KeyValuePair<string, string>("idParceiro", idParceiro),
                        new KeyValuePair<string, string>("codigo", codigo)
                    });

                    var result = await client.PostAsync("cupons/VerificaCupom/", content);
                    if (!result.IsSuccessStatusCode) return null;
                    var resultContent = await result.Content.ReadAsStringAsync();
                    return JsonConvert.DeserializeObject<List<Cupom>>(resultContent);
                    //Debug.WriteLine(resultContent);

                }