WHMCS Local API上的AddOrder问题

I'm adding a new order using WHMCS local API. Everything works fine except custom fields.

$command = 'AddOrder';
$postData = array(
    'clientid' => $client_id,
    'pid' => array($product_id),
    'billingcycle' => array('monthly'),
    'customfields' => array(base64_encode(serialize(array(1 => $site_id)))),
    'paymentmethod' => 'stripe',
);
return localAPI($command, $postData);

My custom field ID is 53 but I set the key to 1 because of tutorials. Also I tried 53 as key array(base64_encode(serialize(array(53 => $site_id)))) but nothing changed.

Do you have any suggestion?

Try this:

 $command = 'AddOrder';
 $postData = array(
'clientid' => $client_id,
'pid' => array($product_id),
'billingcycle' => array('monthly'),
'customfields[0]' => array(base64_encode(serialize(array(1 =>     $site_id)))),//changes here
'paymentmethod' => 'stripe',
);
return localAPI($command, $postData);ode here