/**
 * Callback function for the 'webmention_endpoint_form' filter.
 *
 * @param mixed $WEBMENTION_PLUGIN_DIR.'templates/endpoint-form.php' 
 * @return mixed The filtered value.
 */
function my_webmention_endpoint_form_callback( WEBMENTION_PLUGIN_DIR.'templates/endpoint-form.php' ) {
    // Your code here.
    return WEBMENTION_PLUGIN_DIR.'templates/endpoint-form.php';
}
add_filter( 'webmention_endpoint_form', 'my_webmention_endpoint_form_callback' );

apply_filters( 'webmention_endpoint_form', WEBMENTION_PLUGIN_DIR . 'templates/endpoint-form.php' )