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

apply_filters( 'webmention_schedule_message', esc_html__( 'Webmention is scheduled', 'webmention' ) )