How to enable Access-challenge in Free radius

I have installed Free radius in a windows box and using this radius server to authenticate into my application. I now come across with another need like in addition to authentication i need to overhead with one more option ie. "Access challenge". Can anyone tell me in which file & what needs to be configured so that my radius server can put back a Access-challenge request towards my application so upon providing replying for this challenge request from my application, my radius server should allow me to login in.  Regards, Sridhar