RestSharp AddHeader not working

我正在使用c#和RestSharp 105.2.3构建一个简单的REST/Json windows客户端。一切都工作得很好，但是我指定的额外HTTP头似乎没有根据Wireshark跟踪添加。我错过了什么，或者有一个底层的HTTP方法，我应该使用?

代码很简单，如下所示，我无法在跟踪中看到requesttid头:

    var client = new RestClient("http://example.com");
    client.Authenticator = new HttpBasicAuthenticator("username", password");
    var requestParams = new RequestItem
    { Parameter1 = "test1", Parameter2 = "test2" };
    var request = new RestRequest("/sample/", Method.POST);
    request.AddHeader("RequestId", "value");
    request.RequestFormat = DataFormat.Json;
    request.Parameters.Clear();
    var requestParamsJson = request.JsonSerializer.Serialize(requestParams);
    request.AddParameter("application/json; charset=utf-8", requestParamsJson, ParameterType.RequestBody);
    IRestResponse<ResponseItem> response = client.Execute<ResponseItem>(request);