Unable to post to custom fields

[Call Center Tech]

Hello,

I am having issues posting data in custom fields using api. Here is my php file.

<?php

// API Connection Details
$whmcsUrl = "https://my.callcentertech.net/";

$username = "hahahahahaha";
$password = "hohohohohoho";

// Set post values
$postfields = array(
'username' => $username,
'password' => $password,
'accesskey' => 'itsasecret',
'serviceid' => '107',
'status' => 'Active',
'action' => 'UpdateClientProduct',
'customfields' => base64_encode(serialize(array("4"=>"testing"))),
'responsetype' => 'json',
);

// Call the API
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $whmcsUrl . 'includes/api.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($postfields));
$response = curl_exec($ch);
if (curl_error($ch)) {
die('Unable to connect: ' . curl_errno($ch) . ' - ' . curl_error($ch));
}
curl_close($ch);

// Decode response
$jsonData = json_decode($response, true);

// Dump array structure for inspection
var_dump($jsonData);

============

My field name is : superadmin|VICIdial Super Admin

I am able to update the status but unable to post to custom fields. I ran getproductdetails and here is how the custom field appears.

[2]=>
array(4) {
["id"]=>
int(215)
["name"]=>
string(20) "VICIdial Super Admin"
["translated_name"]=>
string(20) "VICIdial Super Admin"
["value"]=>
string(0) ""

Please assist..