How to Hide the "WEN Post Expiry Notification" Admin Menu

function plt_hide_wen_post_expiry_notification_menus() {
	//Hide the "Settings → WEN Post Expiry Notification" menu.
	remove_submenu_page('options-general.php', 'wpen');
}

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

Where do I put this code?