Add iOS and macOS devices manually to Profile Manager on Mac
After you add users to Profile Manager, add devices to Profile Manager. Then you can associate the devices with specific users.
Placeholder device records are optional if you let users enable device management on their own using the user portal or if you enable enrollment profiles to be used on unknown devices.
In the Profile Manager
sidebar, select Devices, click the Add button 
, then choose Add Placeholder in the pop-up menu.Choose iOS or macOS from the Device Type pop-up menu, then enter a name for this device.
Choose an identifier in the pop-up menu, then enter the relevant information (at least one of these is required):
You can find these identifiers for iOS devices in Settings > General > About, and in iTunes in macOS 10.14 and earlier and the Finder in macOS 10.15 and later when the device is connected. On a Mac, choose Apple menu > About This Mac.
Serial Number: The Apple serial number of the device.
UDID: The Unique Device Identifier of the device, which is used for enrolling the device into Profile Manager.
IMEI: The International Mobile Equipment Identity of an iOS device. This value is unique to each device, regardless of the SIM or eSIM numbers.
MEID: The Mobile Equipment Identifier of the iOS device. This value is also unique to each device, regardless of the SIM or eSIM numbers.
Click Add to register the device.
When the user connects to the Profile Manager using this device, the following is installed:
An enrollment profile
The user’s default configuration profile, if the device is already assigned to a user
Configuration profiles you assigned to this device or its groups
Information about the device, such as installed apps, is also recorded.