WPF返回403 Forbidden错误但邮递员没有

Question

我正在尝试使用RESTful Web API。当我使用Postman时我连接成功，但是当我从WPF连接时出现403错误。这是来自WPF的异步函数：

using (var client = new HttpClient())
{

    client.BaseAddress = new Uri("https://uat.fitygrouponline.com/onlineopeningservice/");
    client.DefaultRequestHeaders.Accept.Clear();
    client.DefaultRequestHeaders.Add("AppId", "12345");
    client.DefaultRequestHeaders.Add("AppKey", "123456");
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));


    long re = 2017;

    string date = String.Format("{0:yyyy-MM-dd}", Convert.ToDateTime(newdat.Text));//check this format

    Dictionary<string, object> para = new Dictionary<string, object>();
    para.Add("Title", title.Text);//required
    para.Add("Complete ", "Yes");//required
    para.Add("ReferenceNo", re);//required
    para.Add("Resident", "Yes");

    var jsonString = JsonConvert.SerializeObject(para);
    var resp = await client.PostAsync("api/opening/create", new StringContent(jsonString, Encoding.UTF8, "application/json"));


    if (resp.IsSuccessStatusCode)
    {
        // use this if you want object type returned
        content = JsonConvert.DeserializeObject<dynamic>(Encoding.UTF8.GetString(resp.Content.ReadAsByteArrayAsync().Result));

        prog.IsActive = false;
        cont.Content = content.responseMessageField;
        MessageBox.Show(content.NumberField);
    }
    else
    {
        MessageBox.Show(resp.ReasonPhrase);
    }
}

我检查resp.IsSuccessStatusCode时收到403错误。我该如何解决这个问题？