How to Hide the "Uji Countdown" Admin Menu

function plt_hide_uji_countdown_menus() {
	//Hide the "Settings → Uji Countdown" menu.
	remove_submenu_page('options-general.php', 'ujicountdown');
}

add_action('admin_menu', 'plt_hide_uji_countdown_menus', 11);

Where do I put this code?