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[Route("api/formHTML/{id}/{code}/{value}")]
        public IEnumerable<string> Get(int id, int code, int value)
        {
            return new string[] {"<html><head></head><body>Test</body></html>"};
        }

protected void Application_Start(object sender, EventArgs e)
    {
         GlobalConfiguration.Configuration.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}"
        );
    }

GlobalConfiguration.Configure(WebApiConfig.Register);

public class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            config.MapHttpAttributeRoutes();
            config.Routes.MapHttpRoute(
              name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = System.Web.Http.RouteParameter.Optional }
           );
        }
    }