Add a captcha using program code

i have a form which allows users to input information. users can later edit the info they input using the same form.

i would like to add a captcha to the form when users input information (create a new post), but the captcha challenge is not necessary when users later edit the info.

so i think i have to add the captcha challenge using program code instead of the admin interface. how can i do so?

thanks a lot.