这里有一种方法可以为新帖子添加一个已经添加并可见的自定义字段：

/**
 * Preset a custom field for new posts
 *
 * @link http://wordpress.stackexchange.com/a/200554/26350
 */
add_action( \'save_post_post\', function( $post_ID, $post, $update )
{
    if( 
            is_a( $post, \'\\WP_Post\' )
        &&  \'auto-draft\' === $post->post_status
        &&  post_type_supports( $post->post_type, \'custom-fields\' ) 
        && \'0000-00-00 00:00:00\' === $post->post_date_gmt 
        && $post_ID > 0
        && ! $update
    )
        add_post_meta( $post_ID, \'wpse_custom_field\', \'123\' );

}, 10, 3 );

然后，我们将在添加新帖子屏幕上看到：

/**
 * Preset a custom field for new posts
 *
 * @link http://wordpress.stackexchange.com/a/200554/26350
 */
add_action( \'save_post_post\', function( $post_ID )
{
    if( \'auto-draft\' === get_post_status( $post_ID )
        &&  post_type_supports( get_post_type( $post_ID ), \'custom-fields\' ) 
    )
        add_post_meta( $post_ID, \'wpse_custom_field\', \'123\' );
} );