LongTerm capital Gain exemption on Agreement of Sale document

I have bough one apartment on 2013 for 32 lakhs and sold the same flat in 2021 Aug for 62.5 lakhs-in Pune.

When I calculate the Long Term capital gain tax then it is coming around 18 Lakhs - Hence I need to pay 20% as Long term capital gain which is around 3.6 Lakhs.

Now I Have booked a new flat at Hyderabad , which is under construction and will be handed over after 3 years.

I have done a Agreement of Sale with the builder now , but actual Registration-Sale deed will happen during handover.

My Query is How Can I claim Long term capital gain exemtion?

IS Agreement of sale considered or DO I need to do registration to claim the exemption?

Please Help.

Thanks,

Samar Pradhan

TaxPayer