How to Hide Darkify Admin Menus

function plt_hide_darkify_menus() {
	//Hide the "Darkify" menu.
	remove_menu_page('darkify');
	//Hide the "Darkify → Darkify" menu.
	remove_submenu_page('darkify', 'darkify');
	//Hide the "Darkify → 👑 Upgrade to Pro!" menu.
	remove_submenu_page('darkify', 'https://darkifywp.com/pricing');
}

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

Where do I put this code?