[HttpPost]
   public object Post(string portfolioAPIKey, DateTime? lastCreativeUTC)
   {
           ......
   }

config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );

User-Agent: Fiddler
  Host: xx.com
  Content-Type: application/json

{"portfolioAPIKey":"6B026A0F-F270-4DBF-B84F-CE41F30099A6","lastCreativeUTC":"2014-12-19T12:17:17.8672363Z"}