Open Wrt with nodogsplash

I am using OpenWrt in my TP_Link TL-WRD3600 router. I have already set up a hotspot plugin (Nodogsplash) on my router. Now I need to allow access to some website before authentication on the splash page. But I need to allow this website using its domain name, eg: example.com, because my splash page logs in to an external website. FirewallRuleSet preauthenticated-users { FirewallRule allow tcp port 80 to example.com } But clients are not allowed to navigate to this page before authentication has completed.