公司内部系统asp.net要一个接口与钉钉对接实现向用户发送消息如何做到

protected void SendMessageToDing(string jsonData)
    {
        string Url = Urls.send_to_conversation + "?" + accessToken;
        string param="{\"sender:\",\"cid:\",\"msgtype:\"\"text\",\"text\":{\"content\":\"张三的请求\"}}";
        System.Net.HttpWebRequest  request =(System.Net.HttpWebRequest)WebRequest.Create(Url);
        request.Method = "post";
        request.ContentType = "application/json;charset=UTF-8";
        byte[] payload;
        payload = System.Text.Encoding.UTF8.GetBytes(param);
        request.ContentLength = payload.Length;
        Stream writer = request.GetRequestStream();
        writer.Write(payload, 0, payload.Length);
        writer.Close();
}
        
    }
怎么获取cid？