Adding devices to Apple Business Manager manually is not easy anymore .Simplest option is to use the reseller number or customer number to get the devices added or use Apple Configurator to add them into Apple Business Manager.
Lets see how to add / remove devices to Apple Business Manager integrated with Endpoint manager. Connect the Apple Device to Apple MAC desktop or laptop , Make sure Apple Configurator 2 is installed.
Download and install Apple Configurator
Lets create a Wi-Fi profile so that the device can connect to internet upon boot.
Apple Configurator 2 – New Profile – Wi-Fi
For – An unexpected error has occurred – Provisional enrollment failed – The Cloud configuration server is unavailable. Make sure Internet sharing is ON on MAC. So that it goes through successfully.