How to Hide the "Datepicker NMR" Admin Menu

function plt_hide_custom_datepicker_nmr_menus() {
	//Hide the "Settings → Datepicker NMR" menu.
	remove_submenu_page('options-general.php', 'nmr-datepicker');
}

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

Where do I put this code?