Docusign /oauth/token端点返回页，而不是json与承载c#

我在DocuSign API中经历了OAuth2过程，我使用官方文档遵循了所有步骤，但是当我试图执行请求以获得AccessToken时，我收到了一个HTML作为响应，表示"DocuSign暂时不可用"。请稍后再试一次。"虽然http响应是200(OK)，但奇怪的是，当我在Postman上使用相同的值进行测试时，我得到了正确的响应。

这是我的代码

public static DocuSignBearerToken GetBearerToken(string AccessCode, bool RefreshToken = false)
    {
        string AuthHeader = string.Format("{0}:{1}", DocuSignConfig.IntegratorKey, DocuSignConfig.SecretKey);
        var client = new RestClient("http://account-d.docusign.com");
        client.Authenticator = new HttpBasicAuthenticator(DocuSignConfig.IntegratorKey, DocuSignConfig.SecretKey);
        var request = new RestRequest("/oauth/token", Method.POST);
        request.AddHeader("content-type", "application/x-www-form-urlencoded");
        request.AddHeader("authorization", "Basic " + Base64Encode(AuthHeader));
        if(!RefreshToken)
            request.AddParameter("application/x-www-form-urlencoded", string.Format("grant_type=authorization_code&code={0}", AccessCode), ParameterType.RequestBody);
        else
            request.AddParameter("application/x-www-form-urlencoded", string.Format("grant_type=refresh_token&refresh_token={0}", AccessCode), ParameterType.RequestBody);
        IRestResponse response = client.Execute(request);
        var responseString = response.Content;
        DocuSignBearerToken Result = JsonConvert.DeserializeObject<DocuSignBearerToken>(responseString);
        return Result;
    }