Hey Hanneke,
You'll have to do 2 things. First in the settings under template you'll have to require the user to submit an email (unless you are dealing with only registered users -> we'll use the e-mail that they've provided in their account).
After that you'll have to make a template override for default_form.php.
compojoom.com/support/documentation/ccomment/ch08#id442946
You'll have to change line 172 from
<input type="checkbox" value="1" {{#user.notify}}checked="checked"{{/user.notify}} name="notify"
name="ccomment-notify" />to
<input type="checkbox" value="1" {{#user.notify}}checked="checked"{{/user.notify}} name="notify"
name="ccomment-notify" checked />
Regards,
Daniel
