Alright I figured this out. The version of RestSharp I have (NUGET), apparently does not support the method above. Also the Resource property is the one that should be getting the url that is going to be replaced so the final code is something like this.
string _baseUrl = "www.test.com";
RestClient client = new RestClient(_baseUrl);
RestRequest restRequest = new Request();
restRequest.Resource = "/{someToken}/Testing";
restRequest.AddParameter("someToken", theToken , ParameterType.UrlSegment);
This piece of code works with the version I got from NUGET