Adding Devices to my Apple ID and iCloud

See the article Check your Apple ID device list to see where you're signed in - Apple Support

Add a device to the list

To add a device to the list, you need to sign in with your Apple ID. After you sign in on a device with your Apple ID, that device appears in the list. Here's where you need to sign in .

• For macOS or Windows 10 and later, sign in to iCloud on the device. 

Note : you have to use same id and password and exactly the same account credentials that is functional ( signed in ) in iPad with MacBook Pro and iMac .

The old Apple ID that is not signed in the device should be remembered , the password can be reset if all credentials that were used in the account using the link https://iforgot.apple.com/appleid#!&section=appleid

Apple support don't keep records of Apple ID address , as account holder can change it , however if mails that were send from the Apple ID address , that can be asked from the recipients If you forgot your Apple ID email address or phone number - Apple Support

If you forget your Apple ID or password - Apple Support

How do you get your old email and password to sign in to add a device if you don’t remember?