Show success message using jQuery AJAX after form submission in ASP.Net

In a post of a webform application asp.net 4.0

i save a user order then in case of success i would add a div with success

but i can't 

$.ajax({
    url: 'api/Order',
    data: JSON.stringify(order),
    type: 'POST',
    contentType: 'application/json',
    dataType: 'json',
    beforeSend: function () {
      ...
    },
    success: function (result) {
        $('#message').append("<div class='alert alert-success'><i class='fa fa-check'></i> Order successfully added!</div>");                   
    },
    complete: function () {
        $('#myModal').modal('hide');
    },
    failure: function (jqXHR, textStatus, errorThrown) {
       ...
    }
});

    public void Post([FromBody] ComponibileCreate model)
    {
        if (!ModelState.IsValid) return;
        string utente = HttpContext.Current.User.Identity.Name;       
        var taskWork = Task.Run(() => SaveOnDatabase(model, utente));
        //Do other work...
        var codici = String.Join("-", model.Righe.Select(i => i.Codice));
        var simboli = String.Join("*", model.Righe.Select(i => i.Descrizione));

        SendMail(utente, codici, userMail);
        SendMailToUser(utente, ragsoc, simboli, userMail);

        taskWork.Wait();//Then wait for thread finish
    }

    private void SaveOnDatabase(ComponibileCreate model, string utente)
    {
        var ordine = new COM_ORDINI
        {
            Data = DateTime.Now,
            Codcli = utente,
            Totale = model.Totale
        };

        foreach (var item in model.Righe)
        {
            var righe = new COM_RIGHE
            {
                COM_ORDINI = ordine,
                Codice = item.Codice,
                Descrizione = item.Descrizione,
                Prezzo = item.Prezzo
            };
            ordine.COM_RIGHE.Add(righe);
        }

        _ctx.COM_ORDINI.Add(ordine);
        _ctx.SaveChanges();
    }

I recive no error on console and in database all works