WP-Functions /Add custom field automatically when post or page is publish

new_gist_file.php

add_action('publish_page', 'add_custom_field_automatically');
add_action('publish_post'. 'add_custom_field_automatically');
function add_custom_field_automatically($post_ID) {
    global $wpdb;
    if(!wp_is_post_revision($post_ID)) {
        add_post_meta($post_ID, 'FIELD_NAME', 'CUSTOM VALUE', true);
    }
}