How to configure Captive Portal for PPTP-Connections ?

Hi,

i have configured Monowall for PPTP. When am connecting with a client to  
this server
i have automatically access to the internet. But i wish to have a captive  
portal.

Does it works ? If yes what i have to configure then ?

Cause if it works, i can use my Hotspot without any encryption and all  
users are
logging in with PPTP and every connection is encrypted. Is it right?

Thank you for your answers beforehand.

Greetings from Germany

Wolf