My orders are not being pushed to Virtual Systems Midnight and are failing with error "invalid customer id"?

PRESSERO > *Integration information & Supported Services
There is additional setup that needs to be done to the site users before the orders will push to Midnight without error. Go to Site > User Management > Site Users and then to the Embedded Services tab. The External ID field needs to be populated for any Midnight users. Please see 7) User Configuration on this article for more information.