Make captive portal work on DD WRT even when hotspot has no active

We are using DD WRT firmware on D-Link AC 3200. The router is use for creating a offline hotspot. The users connect to the hotspot to browse a website hosted locally on server connected to the hotspot directly. We have setup dnsmasq on the router to redirect the website url ([login to view URL]) to the ip address of the server. We want to configure captive portal on the router such that any user who connects to the hotspot is automatically redirected to [login to view URL] url.