set-jetpack-publicize-default-message.php

<?php
/**
 * Set Jetpack Publicize Default Message
 *
 * @package     SetJetpackPublicizeDefaultMessage
 * @author      Bernhard Kau
 * @license     GPLv3
 *
 * @wordpress-plugin
 * Plugin Name: Set Jetpack Publicize Default Message
 * Plugin URI: https://kau-boys.com
 * Description: Sets the default sharing message
 * Version: 0.1
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.com
 * Text Domain: set-jetpack-publicize-default-message
 */

/**
 * Save a post metadata for a default sharing title if none is set.
 */
function set_jetpack_publicize_default_message() {
	global $post;

	if ( ! empty( $post->ID ) ) {
		if ( ! $wpas_title = get_post_meta( $post->ID, '_wpas_mess', true ) ) {
			/* translators: %s: the name of the blog */
			$custom_message = sprintf(
				__( 'Check out this new awesome blog post on %s!', 'set-jetpack-publicize-default-message' ),
				get_bloginfo( 'name' )
			);
			update_post_meta( $post->ID, '_wpas_mess', $custom_message );
		}
	}
}

add_action( 'post_submitbox_misc_actions', 'set_jetpack_publicize_default_message' );