Yeah! You can select your category as EWS for an examination, even if you don't have the certificate at that moment, but you should get it by the time of counselling. Now during the counselling if you won't able to produce your category certificate, then you have to appear for the UR & you have to provide a declaration as per the rules & norms suggested by the examination conducting authority you are appearing for.

I hope my answer provides you some insight. Wish you a great eve!