To get the full URL use RestClient.BuildUri()
Specifically, in this example use client.BuildUri(request)
:
RestClient client = new RestClient("http://www.some_domain.com");
RestRequest request = new RestRequest("some/resource", Method.GET);
request.AddParameter("some_param_name", "some_param_value", ParameterType.QueryString);
IRestResponse<ResponseData> response = client.Execute<ResponseData>(request);
var fullUrl = client.BuildUri(request);