2
In the file include/registerform.php you see lines like this:
$reg_form->addElement(new XoopsFormText(_US_WEBSITE, "url", 25, 255, $url));
If you want to make a form element required, change the last bit of the line, like this:
$reg_form->addElement(new XoopsFormText(_US_WEBSITE, "url", 25, 255, $url), true);