I am deveoping a small plugin that needs to alter the registration and comment forms to add 4 hidden fields that have data inside that I gather from the configuration panel, and when the form is submitted, a js onlick event must be called to send those 4 hidden field data (it's a tracking software similar to google analytics).
So far I've built the xml manifest and xml field structure following this link and this link tutorials.
The first issue is that the fields are hidden, so the data is not entered by the user. I need to fill those fields data with:
<?php echo $params->get('my_param'); ?>
but on those tutorials I don't see any example on how to fill the fields when the form is loaded. I'm also lost on how to attach a js onclick event to the submit form. It is enough to implement onAfterRender()?
EDIT: The registration form is the one where an user register to the joomla cms.
The comment form is any comment form added by a third party extension below posts and pages.