Hello Pankaj, 

As per the question asked by you, if you have already passed the mathematics exam appeared through online mode, you can put mathematics as a qualifying subject. But as you have already received the degree for your HSC conducted in 2018 without having mathematics as a subject, you can't add mathematics there as you have already passed the qualifying degree. Hence you can do one thing, first get a passing certificate for the mathematics paper and you can apply next year adding it to your list 12th degree subjects. 

I hope this answer helps you. All the best. !