HOW TO ADD IPHONE TO ACCOUNT

Sign into it with your ID. Read this support article to learn how to sign in with your Apple ID on different devices--> Sign in with your Apple ID - Apple Support

For an Apple Mobile device:

1. Open the Settings app.

2. Tap Sign in to your [device].

3. Enter your Apple ID and password.

4. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.