how to add custom meta field in custom post type
function global_notice_meta_box() {
$screens = array( 'post', 'page', 'book' );
foreach ( $screens as $screen ) {
add_meta_box(
'global-notice',
__( 'Global Notice', 'sitepoint' ),
'global_notice_meta_box_callback',
$screen
);
}
}
add_action( 'add_meta_boxes', 'global_notice_meta_box' );