Skip login on MVC 3 application (SSO)

Question

I have an MVC application which is using forms authentication. In the global.asax file, whenever trying to get into a site, you'll be redirected to login form:

public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            "Default", // Route name
            "{controller}/{action}/{id}", // URL with parameters
            new { controller = "Login", action = "Login", id = UrlParameter.Optional } // Parameter defaults
        );
    }

I also have another application which should call this application in an iframe. The iframe should have a link of one of my inner pages and shouldn't go through the login page (the other application sends the username, and then I have an authentication method), and get directly to the inner page, but I can't seem to get the url of the inner page.

I tried using System.Web.HttpContext.Current.Request.UrlReferrer but it appears null

How can I get this info and skip the login page?

Thanks