Thanks for the clarification, which however is a bit disappointing as I was looking forward to this feature in order to escape all traffic/devices to be routed through VPN. It would be very tedious to figure the IP for each content delivered to a tunnelled VPN.The router has no way of knowing what URLs are in that webpage you are loading, so there's nothing the routing table can do about it.
You will have to define a whole subnet, or multiple rules. Domain names are not supported, because a routing table needs a specific IP or subnet - it has no way of doing any name resolution at routing time (which would be half-useless anyway, as your browser might have resolved a different IP than what the router would resolve, if that hostname uses a round-robin entry for instance).
What I am not getting is how the browser would eventually resolve an IP differently than the router since the browser is relying on the router for the task? And why a domain name resolution at routing time is not possible?