C#HttpClient放置/张贴谷歌日历API

我正在尝试执行PUT to Google Calendar API v3以添加与会者，但每次我都会收到400 BAD REQUEST的响应。我简化了它，只是尝试更新最大出席人数，看看是否需要，但没有骰子。。。

        public void AddAttendeeToEvent(string calendarId, string eventId, string attendeeEmail)
        {
        try
        {
            var httpClient = new HttpClient();
            httpClient.DefaultRequestHeaders.Add("Authorization", "Bearer " + AuthenticationHelper.Instance.Token);
            var baseUrl = String.Format("https://www.googleapis.com/calendar/v3/calendars/{0}/events/{1}", calendarId, eventId);
            //var postData = string.Format ("attendees[].email={0}", attendeeEmail);
            var postData = "maxAttendees=100";
            var content = new StringContent(postData, Encoding.UTF8, "application/x-www-form-urlencoded");
            var response = httpClient.PutAsync(baseUrl, content).Result;
            response.EnsureSuccessStatusCode ();
        }
        catch(Exception ex)
        {
            string s = "";
        }
    }

注意-这是在Xamarin，没有可用的客户端库。现在必须手动完成。。。有关这方面的文件非常糟糕。