How to add Purchase Order to a Record?

Hello Guys,

How can we add Purchase Order to a certain Record?

I have this code below and its not working. I'm having an "Invalid entity reference key"

$customer_id = $this->input->post('internalID');
 
// create array of fields
$purchaseOrderFields = array (
    'entity'  => array ('internalId' => $customer_id),
    'itemList'  => array (
                'item' => array (
                    array('item' =>
                                                    array('internalId' => 116),
           'quantity' => 1)
                                                 )
                        )
);

// create purchase order record
$purchaseOrder = new nsComplexObject('PurchaseOrder');

// set fields
$purchaseOrder->setFields($purchaseOrderFields);

// this will create a purchase order with 1 line item
$addResponse = $myNSclient->add($purchaseOrder);

Can anyone help me figure out what's wrong with my codes? I really need your help guys. Thanks.