how to add iphone devise to my account

To what account exactly?

To add an iPhone to the My devices list, you need to sign in to iCloud on the iPhone in question.

Sign in with your Apple ID - Apple Support

Buy the iPhone and when setting it up use the same Apple ID and password that you use on your other Apple devices.

If that is not what you meant by your question perhaps you can explain in more detail.

You log into your Apple ID on the iPhone. Go to Settings>your name>iTunes & App Store and log in with your Apple ID.