add_filter(
   'notify_user_about_keyword_post',
    function(
        $notify_user,
        $user,
        $post,
        $keyword
    ) {
        // Your code here.
        return $notify_user;
    },
    10,
    4
);

apply_filters( 'notify_user_about_keyword_post', $notify_user, $user, $post, $keyword )