To claim a device by only the MAC you must first claim the device in RPS.
The device must not be claimed by any other organization.
Navigate to RPS Management and click + Add
Then add the MAC and select the server, click Save.
Next, in RPS Management, check the box of the newly added MAC(s), click sync. A popup will appear, select the site to add the device(s) to, click Confirm.
Now if you search for the MAC(s) in YMCS they will appear.