Hello dear

You must received an email and message on your phone from where you can access your id.

But if you are still not able to retrieve it you can call them or email them on this mail :

fic.fd@pilani.bits-pilani.ac.in

Mail them from your register mail id and also you can contact them from the same number , you will find the contact details from the official website of BITSAT.

Hope they will definitely reply and help you out.

Good luck