FedEx - Add P_O_NUMBER, INVOICE_NUMBER, CUSTOMER_REFERENCE & DEPARTMENT_NUMBER while label printing

functions.php

add_filter('wf_fedex_request','wf_fedex_label_request_reff_callback', 10, 2 );
function wf_fedex_label_request_reff_callback($request,$order){
    $po_number = 'x';
    $inv_number = 'x';
    $customs_duties_payer = 'SENDER';
    
    $reff = array();
    $reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'P_O_NUMBER', 'Value' => $po_number );
    $reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'INVOICE_NUMBER', 'Value' => $inv_number );
    $reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'CUSTOMER_REFERENCE', 'Value' => $order->id );
    $reff['CustomerReferences'][] = array( 'CustomerReferenceType' => 'DEPARTMENT_NUMBER', 'Value' => 'Bill Duties : '.$customs_duties_payer);
    
    foreach ($request['RequestedShipment']['RequestedPackageLineItems'] as $key => $value) {
           $request['RequestedShipment']['RequestedPackageLineItems'][$key] += $reff; 
    }
    return $request;
}