The Device Enrollment Program (DEP) 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), you should see those serial numbers appear in the DEP Devices section of Jamf Now.
To enroll a DEP 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.