POST is not receiving JSON

I need some help with running a POST. Well, when I try to call my API ( link ) to execute a Post, I can not pass the JSON for her.

The following is the API used:

// POST: api/Teste
public void Post([FromBody]string json)
{
    ClassRetorno cr = new ClassRetorno();

    if (string.IsNullOrEmpty(json))
    {
        cr = JsonConvert.DeserializeObject<ClassRetorno>("{\"Id\":1,\"Nome\":\"From Api\",\"Email\":\"[email protected]\"}");
    }
    else
    {
        cr = JsonConvert.DeserializeObject<ClassRetorno>(json);
    }

     mysql = new ClassMySQL();
     if (mysql.conectar())
     {
         mysql.insert("teste", "id, nome, email", cr.Id + ",'" + cr.Nome + "','" + cr.Email + "'");
     }
}

In short: If the string Json is null or empty, it will save to the bank (Id: 1, Name: From Api, Email: [email protected])

If not, it will save according to the received parameter.

This is the method that runs Post:

public async Task Posting(ClassRetorno item)
{
    using (var client = new HttpClient())
    {
        var uri = new Uri("http://www.afectus.com.br/api/teste");
        var json = JsonConvert.SerializeObject(item);
        var content = new StringContent(json, Encoding.UTF8, "text/json");
        HttpResponseMessage response = new HttpResponseMessage();
        response = await client.PostAsync(uri, content);
        if (response.IsSuccessStatusCode)
        {
            await DisplayAlert("Alert", "Post executado!", "Ok");
        }
        else
        {
            await DisplayAlert("Alert", "Post não executado!", "Ok");
        }
    }
}

If anyone knows the possible cause of this problem, or at least, can give some guidance, I will be grateful.

Obs : I used the Postman tool to test the API and it still did not work.