How to make PO Number required

Hello.

How can I make the PO number required with this plugin?
I thought the following may work in my functions.php but not luck

add_filter( 'woocommerce_order_fields', 'po_require_po_field');
function po_require_po_field( $fields ) {
$fields['woocommerce-checkout-payment']['igfw_purchase_order_number']['required'] = true;
return $fields;
}