Username field blank | whmcs

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Hey guys,
So, I've added CentovaCast to my WHMCS and for some reason, when someone places an order it doesn't assign a username. I followed this guide linked below, and despite creating a "username" custom field, it's still not doing anything.

Any help would be highly greatful.
centova.com/doc/cast/installation_manual/09_billing_integration/whmcs