How to update the client custom field in whmcs

I have to update the client custom field in WHMCs. I tried with the following code.

    $user=Whmcs::UpdateClient([
    'clientid'=>$login['userid'],
    'customfields' => 
     base64_encode(serialize(array("customfield18"=>"12345"))),

      ]);

From the above code I have to update the client custom field in WHMCS.

via Chebli Mohamed