Dynamic modify LoginUrl for FormsAuthentication

gistfile1.cs

            System.Configuration.Configuration configuration =
                   WebConfigurationManager.OpenWebConfiguration("/YourSite");

            // Get the external Authentication section.
            AuthenticationSection authenticationSection =
                (AuthenticationSection)configuration.GetSection(
                "system.web/authentication");

            // Get the external Forms section .
            FormsAuthenticationConfiguration formsAuthentication =
                authenticationSection.Forms;

            formsAuthentication.LoginUrl = "http://google.com";