/**
 * Callback function for the 'webmention_endpoint' filter.
 *
 * @param mixed $rest_url 
 * @return mixed The filtered value.
 */
function my_webmention_endpoint_callback( $rest_url ) {
    // Your code here.
    return $rest_url;
}
add_filter( 'webmention_endpoint', 'my_webmention_endpoint_callback' );

apply_filters( 'webmention_endpoint', get_rest_url( null, '/webmention/1.0/endpoint' ) )