How to Hide the "Pro Missa" Admin Menu

function plt_hide_promissa_menus() {
	//Hide the "Settings → Pro Missa" menu.
	remove_submenu_page('options-general.php', 'promissa/promissa.php');
}

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

Where do I put this code?