C#API调用不适用于HttpWebRequest,但适用于Postman

我有以下邮递员要求:

按预期返回URL:

我试着用一个。Net Core 2.0应用程序,代码如下:

static void Main(string[] args)
{
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

    var httpWebRequest = (HttpWebRequest)WebRequest.Create("https://epaper.20minuten.ch/index.cfm/epaper/1.0/getEditionDoc");
    httpWebRequest.ContentType = "application/json";
    httpWebRequest.Method = "POST";

    var serializer = new Newtonsoft.Json.JsonSerializer();
    using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
    {
        using (var tw = new Newtonsoft.Json.JsonTextWriter(streamWriter))
        {
            serializer.Serialize(tw,
                new
                {
                    editions = new[]
                    {
                            new
                            {
                                defId = "648",
                                publicationDate = "2018-03-06"
                            }
                    },
                    isAttachment = true,
                    fileName = "Gesamtausgabe_Lausanne_2018-03-06.pdf"
                });
        }
    }
    var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
    using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
    {
        var responseText = streamReader.ReadToEnd();
        Console.ReadLine();
    }
}

但我收到500个错误。我错过了什么?