For information, here is the code for the Akeeba Release system sample plugin:

<?php
/**
 * Sample plugin to update a paid-for extension
 * released using Akeeba Release System
 *
 * @copyright   Copyright (C) 2005 - 2014 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

/**
 * Handle commercial extension update authorization
 *
 * @package     MyPackage
 * @subpackage  MyPackage.update
 * @since       2.5
 */
class PlgInstallerSamplears extends JPlugin
{
    /**
     * @var    String  base update url, to decide whether to process the event or not
     * @since  2.5
     */
    private $baseUrl = 'http://update.mysite.com/subscribers/com_myextension';

    /**
     * @var    String  your extension identifier, to retrieve its params
     * @since  2.5
     */
    private $extension = 'com_my_extension';

    /**
     * Handle adding credentials to package download request
     *
     * @param   string  $url        url from which package is going to be downloaded
     * @param   array   $headers    headers to be sent along the download request (key => value format)
     *
     * @return  boolean true if credentials have been added to request or not our business, false otherwise (credentials not set by user)
     *
     * @since   2.5
     */
    public function onInstallerBeforePackageDownload(&$url, &$headers)
    {
        // are we trying to update our extension?
        if (strpos($url, $this->baseUrl) !== 0)
        {
            return true;
        }

        // fetch download id from extension parameters, or
        // wherever you want to store them
        // Get the component information from the #__extensions table
        JLoader::import('joomla.application.component.helper');
        $component = JComponentHelper::getComponent($this->extension);

        // assuming the download id provided by user is stored in component params
        // under the "update_credentials_download_id" key
        $downloadId = $component->params->get('update_credentials_download_id', '');

        // bind credentials to request by appending it to the download url
        if (!empty($downloadId))
        {
            $separator = strpos($url, '?') !== false ? '&' : '?';
            $url .= $separator . 'dlid=' . $downloadId;
        }
        return true;
    }
}