Custom Comment_Form Fields don't work

I want to add a few custom fields to my comments_form but despite trying a few different techniques I can’t get them to work, any ideas?

Environment :
Woothemes definition theme + woocommerce 2.1.5

• 1st technique :

– Added a filter for comment_form_default_fields – like so :

function wpsites_modify_comment_form_fields($fields){
    $fields['new'] = '<p>This will be by custom field</p>';
    return $fields;
}

add_filter('comment_form_default_fields','wpsites_modify_comment_form_fields', 20);

Doesn’t add anything, doesn’t throw any errors.

• 2nd technique

– Added the field to $comments_args – like so :

$comments_args = array( 'new'=> '<p>This will be by custom field</p>' );
 comment_form($comments_args);

Doesn’t work.

• 3rd technique

– added the field directly to the comments_form() as a paramter
comment_form(array( 'new'=> '<p>This will be by custom field</p>' ))
and that didn’t work either

How do I get this to work?