Customize jetpack contactform subject

L.S.

As a business we get lots of mails via our Jetpack contact form. To set all mails apart, i prefer a custom subject line. Lets say with the id added.

For now i managed to fix this via directly editing the grunion-contact-form.php where: $subject = apply_filters( ‘contact_form_subject’, “customtext”.$contact_form_subject, $all_values );

Obviously for cleanness i would want to do this via the functions file. Wrote a function and then used add_filter(‘contact_form_subject’.. I seem to be doing something wrong..