Global.asax.cs

public class MvcApplication : System.Web.HttpApplication
{
    ...

    private void WSFederationAuthenticationModule_RedirectingToIdentityProvider(object sender, RedirectingToIdentityProviderEventArgs e)
    {
        // Get the request url.
        var request = HttpContext.Current.Request;
        var requestUrl = request.Url;

        // Build the realm url.
        var realmUrl = new StringBuilder();
        realmUrl.Append(requestUrl.Scheme);
        realmUrl.Append("://");
        realmUrl.Append(request.Headers["Host"] ?? requestUrl.Authority);
        realmUrl.Append(request.ApplicationPath);
        if (!request.ApplicationPath.EndsWith("/"))
            realmUrl.Append("/");
        e.SignInRequestMessage.Realm = realmUrl.ToString();
    }

    ...
}

Programatically changing the WIF Realm based on the originating domain / URL.