This example will add a string to attach to a subscriber as tag.

add_filter( 'frm_mlcmp_tags', 'add_new_tag' );
function add_new_tag( $tags, $data ) {
    if ( $data['subscriber_id'] === '123' ) {
        $tags[] = array(
            'name'   => 'New tag',
            'status' => 'active',
        );
    }

    return $tags;
}

Replace the value (123) by the id of the subscriber to which you want to add the new tag.