/**
 * Callback function for the 'webmention_send_vars' filter.
 *
 * @param mixed $body 
 * @param mixed $post_id 
 * @return mixed The filtered value.
 */
function my_webmention_send_vars_callback( $body, $post_id ) {
    // Your code here.
    return $body;
}
add_filter( 'webmention_send_vars', 'my_webmention_send_vars_callback', 10, 2 );

apply_filters( 'webmention_send_vars', $body, $post_id )