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

apply_filters( 'webmention_server_url', $webmention_server_url, $target )