The tentative application date for IAT is Third week of March,2021 but no notification have been released so you have to wait and keep an eye on the official website to stay updated about the application dates.

You can apply for IAT by this website once the application is released:

http://www.iiseradmission.in/index.html

To know more about IAT you can visit the link provided below:

https://engineering.careers360.com/articles/iiser-admission

I hope this answer helps. All the very best for your future endeavors!