add iphone to my icloud account

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

Check your Apple ID device list to find where you're signed in - Apple Support

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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. 

Simply log in to your device with your Apple ID. That's it. It will get added automatically.

iCloud uses Apple ID credentials.