I think you want something like Cyber cafe hotspot
as these "hotspots" provide internet access if you login through their system with some kind of token (or whatever)
the thing is I don't know how to set it,
here some kind of hostapd
configuration is explained.
Basically your server must "stand" between internet connection (Radius) and clients, to provide them your site first and then if auth. successful they can proceed surfing.