Hey Aspirant, It is not mandatory to enter the roll no. in application form of Jee mains as while the form was being filled some students had their roll no.(s) while others giving the paper first time were not having it. 

This kind of situation occurs every year that is why Jee main authorities don't mark it mandatory.

So, if you won't give them your roll no. if you are giving the mains paper first time and also giving your 12th boards simultaneously then there will be no issue.

Hope you will get it.