How to Hide the "Billingo számla" Meta Box

function plt_hide_billingo_metaboxes() {
	$screen = get_current_screen();
	if ( !$screen ) {
		return;
	}

	//Hide the "Billingo számla" meta box.
	remove_meta_box('custom_order_option', $screen->id, 'side');
}

add_action('add_meta_boxes', 'plt_hide_billingo_metaboxes', 20);

Where do I put this code?