Setting "External reseller URL"
This URL is showed to the customer whenever he opens "My account" dialog in his admin panel and/or he wants to upgrade.
This URL should be valid URL, but you can use following variables in it to identify customer:
{$accountid} - identifies customer which opened external URL.
Example of URL with variables:
https://www.mycustomerportal.com/upgradeForm/{$accountid}/step_one.php
How it looks like on "My account screen"