Adding custom input field (mandatory) to comments/forum posts

Hello all,

I’m not a php programmer so I’m not finding my way around the source code in SPIP (just a sysadm asked to help). I would like to implement anti-spam question (it can be same every time - it is in local language and unpenetrable for spam bots or humans) which must be correctly answered in order to continue adding comment to an article. Can someone help me where to put this lines of code ?

Thank You