@
neverenoughmiles: There are TWO parts for Air Suvidha
1) Self Reporting Form (filled anytime before you board your flight - preferably at least 72 hours before)
2) Exemption Form - This form is forwarded to the state government of your final destination. Some states require that all passengers have mandatory 14 day quarantine. So they do not grant exemptions (Kerala, Karnataka, Odisha). So fill this out if your final destination is anyplace except Karnataka, Kerala or Odisha. This can be filled out anytime prior to landing. As I mentioned in my post, I submitted mine about 4 hours prior to landing in BOM. But no body looked at it anyway. They just want to see a screenshot or paper print out of a negative PCR.
If you don't have a seat number, I would suggest to wait until you have it and then fill in the SRF (Part 1). I don't think the form will let you finish until you provide the seat number.