External captive portal using firewall

Hi,

I have a scenario where my MK is connecting to a remote network thru IPsec. The remote network has a captive portal which is reachable and accessible manually thru browser on a pc connected on my MK. Now I want the captive portal to pop up directly when any client connects. Is it possible thru Firewal/NAT redirect or I need to use hotspot option only. In hotspot option, after editing login.html in MK root directory, my captive portal is not redirecting, rather I get 404 error.

My external captive portal is http://172.16.88.16/captiveportal?page=login. This is accessible manually.

Pls help me which was is best way to make the network work.