Do we have to fill a separate form for IIIT Allahabad?

I know some people from IIIT Allahabad so I think I can shed some light on the admission process there.

There is no need to fill any additional forms for admission to IIIT Allahabad. The admission procedure of IIIT Allahabad is through the CSAB- Central Seat Allocation Board. After the JEE ranks are allowed, you are required to go through the CSAB counseling procedure and opt for IIIT Allahabad if you are interested in it from the list of Institutes given on the portal.

You can find IIIT Allahabad mentioned in the list with all the courses it offers. Hope this helps.