POST使用HttpClient向列表中添加MailChimp API 3.0

我试图发送一个POST请求，将订阅者添加到使用他们的3.0 API的邮件chimp列表。我发送请求，但我得到的错误是一个405 -方法不允许错误，我不明白如何修复。

我正在使用Http客户端发送PostAsync请求。

任何帮助都将是非常感激的。

代码:

        JavaScriptSerializer jss = new JavaScriptSerializer();
        HttpClient client = new HttpClient();
        foreach (MailChimpSubscriberModel subscriber in newSubscribers)
        {
            Subscriber member = new Subscriber();
            member.email = subscriber.Email;
            member.subscribed = "subscribed";
            var url = "https://" + _dataPoint + ".api.mailchimp.com/3.0/lists/" + _defaultListId + "/";
            string jsonString = jss.Serialize(member);
            StringContent stringContent = new StringContent(jsonString);
            stringContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
            client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", _apiKey);
            HttpResponseMessage response = await client.PostAsync(url, stringContent);
            Trace.WriteLine("Response StatusCode: " + (int)response.StatusCode);
        }