It is clearly mentioned in the public notice released by the authority regarding instructions and advisory for the candidates of Neet/Jee Main 2020. The candidate must enter all the details, signed and paste your photograph in the requird place. You should also put yout left thumb impression where it is required before reaching the exam centre. After reaching the exam centre, you'll not be able to do this. So make sure you have done everything required before reaching the exam centre. The admit card must be duly filled in. The left thumb impression must be clear and not smudged. Basically, blue ink is used for thumb impressions in various documents. So you can buy a blue ink pad for left thumb impression. Before putting the thumb impression on the admit card, do the same in some white papers. The thumb impression must be straight and clear. http://medicine.careers360.com/articles/nta-neet-2020-admit-card-undertaking