C#調用HttpClient.SendAsync報錯：System.Net.Http.HttpRequestException:
發送請求時出錯。

var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken);

問題出在SSL/TLS，Windows Server 2012不支持 TLS 1.2 ，需要手動設置

    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;