This option is available only after v3.8.4
To include to the body tag, Your theme must support the wp_body_open hook
How to Check wp_body_open hook support?
- Go to Appearance
- Go to Theme File Editor
- Open header.php from the sidebar
- Search for
- After your code must be as below
<body <?php body_class(); ?>> <?php do_action("wp_body_open"); ?>
You can replace that if doesn’t exist.
This will not affect your theme.