Configurable Options are not being recognized properly between Plesk & WHMCS.
The Activity log in WHMCS Utilities > Logs > Activity Logs show the following message:
CONFIG_TEXT: Module Create Failed - Service ID: 17 - Error: Error code: 1013. Error message: There is no such a service plan
In Utilities > Logs > Module Logs in the API request the added "Plesk " prefix can be seen:
CONFIG_TEXT: <?xml version="1.0" encoding="UTF-8"?><packet version="126.96.36.199">
<!-- Copyright 1999-2016. Parallels IP Holdings GmbH. -->
<name>Plesk 1 Additional Website</name>
WHMCS extension bug #EXTWHMCS-45 which is planned to be fixed in future product updates.
Until the fix became available, use the following steps as a workaround:
Go to Service Plans > Hosting Plans > Add an Add-on and create addon
no sitesin Plesk with empty limits.