Hello Sauravi

You will have to select Code 01 in the NEET 2019 application form. The field which ask for Roll No. of Class 12th can be left blank.
Explanation given for Code 01 is as follows-

 Candidates who will take Class 12 board exam in 2019 and the results of such candidates are awaited must submit this code. However, qualifying the exam is mandatory to be eligible for admission to MBBS/BDS courses.

Good Luck!