See it is dependent on the norms of the counselling you are taking part in. In general, the EWS certificate are asked during the time of counselling. I would recommend you that if your slot booking didn't needed EWS certificate number then obviously you will be able to add the number during the counselling.

The eligibility for the EWS certificate are:

• The annual income of the family of candidate should not exceed 8lakh per annum.
• The family should not hold 5 or more than 5 acres of land.
• The family shouldn't have a residential area more than 1000 sq feet.
• The family shouldn't have a residential plot (in notified municipalities) of an area 100 square yards or more.
• The family shouldn't have residential plot (other than in notified municipalities) of area 200 square yards or more.

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