Apple School Manager or Apple Business Manager allows you to pre-associate your purchased hardware with your Jamf Now account. The first time these devices turn on, they will automatically enroll into Jamf Now.
Once a device has been added to your MDM Server with Apple and linked with Jamf Now (see details here), the device's serial numbers will appear in Jamf Now on the Auto-Enrollment pane, under View Devices.
To enroll a device, turn on the device and connect to Wi-Fi. The built-in Setup Assistant will detect that the device is associated with Jamf Now and the device will automatically enroll into Jamf Now.