How to insert multiple custom fields to whmcs API

I have an issue to insert multiple product custom fields to whmcs using laravel. In this below code pass the $cp value alone. But need to pass the $os,$db and $cp value.

$order_details = Whmcs::AddOrder([
'clientid' => $value,
'paymentmethod' => 'razorpay',
'pid' => $pid,
'domain' => $domain,
'billingcycle' => $billingcycle,
'domaintype' => $domain_reg,
'regperiod' => $domain_regperiod,
'customfields'=>array(base64_encode(serialize($os))),
'customfields'=>array(base64_encode(serialize($db))),
'customfields'=>array(base64_encode(serialize($cp)))
]);

Custom fields are presented in the form as following:

<input name="customfield[5]" type="text">
<input name="customfield[11]" type="text">

And when submitted, the post vars is:

$_POST['customfield'] = array('5' => '', '11' => '' );

So when using AddOrder function the code becomes:

//using Laravel $request
$customFields = base64_encode(serialize($request->input('customfield')));
//or using $_POST array
$customFields = base64_encode(serialize($_POST['customfield']));

$order_details = Whmcs::AddOrder([
'clientid' => $value,
'paymentmethod' => 'razorpay',
'pid' => $pid,
'domain' => $domain,
'billingcycle' => $billingcycle,
'domaintype' => $domain_reg,
'regperiod' => $domain_regperiod,
'customfields'=> $customFields
]);