c# – 使用RESTSHARP时API返回错误

The underlying connection was closed: An unexpected error occurred on
a send.

我已经验证我的客户端ID,密码,用户名和密码是否正确.我能够在没有PowerShell问题的情况下做到这一点.

public string GetTokenForBrightIdea()
{
    RestClient restclient = new RestClient(_uri);
    RestRequest request = new RestRequest() { Method = Method.POST };

    request.AddHeader("Accept","application/json");
    request.AddHeader("Content-Type","application/x-www-form-urlencoded");
    request.AddParameter("grant_type","password");
    request.AddParameter("client_id",_clientId);
    request.AddParameter("client_secret",_clientSecret);
    request.AddParameter("username",_clientUsername);
    request.AddParameter("password",_clientPassword);

    var tResponse = restclient.Execute(request);
    var responseJson = tResponse.Content;
    return JsonConvert.DeserializeObject<Dictionary<string,object>>(
        responseJson)["access_token"].ToString();
}

使用RestSharp进行此操作时我错过了什么？