whmcsTools/servicecode.php

<?php
use Illuminate\Database\Capsule\Manager as Capsule;

// this hook returns $addon->subscriptionid as $addon.code in smarty

if (!defined("WHMCS"))
    die("This file cannot be accessed directly");

add_hook('ClientAreaProductDetailsPreModuleTemplate', 1, function($vars) {
    foreach($vars['addons'] as $addon) {
        $servicecode[$addon['id']] = Capsule::table('tblhostingaddons')
            ->where('id', $addon['id'])
            ->value('subscriptionid');
    }
    return array('servicecodes' => $servicecode);
});