You may have to enter a certain promo code at the time that you register your new account.
The online casino may have different instructions on how to receive your free no deposit bonus. Typically, when you register a new account and redeem a certain promo code or bonus code, you will be given a free casino chip or free slots spins without having to make a real money deposit.
A no deposit bonus is a pretty rare and very special type of promotion offered by several online casino operators.