How to record conversion?

I have a form for website visitors to subscribe to my newsletter. How could I implement a signal to piwik to record and indicate a new subscriber as a conversion?

Example of answer: add an invisible field that triggers a javascript function,

I use a CMS, so adding to the piwik tracking code affects all pages.

Any pointers would be appreciated.

Thanx,

You can call the javascript function piwikTracker.trackGoal( idGoal ); when someone submits your form: http://piwik.org/docs/javascript-tracking/#toc-manually-trigger-a-conversion-for-a-goal