Hello, Kunal!
I tried that before and wasn't working.
I found working solution on http://community.cambiumnetworks.com/t5/Enterprise-E500-E400-Hotspot/Integration-with-external-login-page-for-guess-access/td-p/56611 - you have to attach to POST URL whole query string.
I have another feature request .. When you select click through captive portal, it would be very useful if you could also POST session_timeout parameter with the HTML form and that it could be higher than 86400 seconds.
Thank you!