Apple requires the use ofApple Configurator for iPhoneto manually add Macs into Apple Business Manager or Apple School Manager. The workflow involves placing the target Mac into Device Enrollment mode and then scanning an on-screen image with the Configurator app running on an iPhone. Once completed, the Mac is added to the organization's ABM account with a 30-day provisional period. Apple Configurator for Mac, while useful for restoring and preparing iOS/iPadOS devices, does not support manually adding Macs to ABM. System Settings and Xcode are unrelated to device enrollment workflows. This method is Apple's prescribed approach for organizations acquiring Macs outside normal reseller channels.
References:Apple Configurator User Guide for iPhone - "Add a Mac to Apple Business Manager or Apple School Manager."