In most cases, there’s no need for a bonus code to get a no deposit bonus. If you do need a bonus code, this will be clearly specified in the bonus offer - but for the most part, you just simply create your new account, verify (usually by SMS) and you’ll be credited with the bonus you deserve.
This is usually applied to your account automatically, assuming you’ve met the terms and conditions for the bonus offer, so you don’t have to worry too much about a bonus code in most cases. If you are taking part in ongoing promotions, you may receive a bonus code to take part.