How do I add a device to my Apple ID?

See the following from View and remove your associated devices in iTunes - Apple Support

Your iPhone, iPad, iPod touch, Mac, PC, or Android phone automatically associate with your Apple ID and iTunes when you:

• Sign in to Apple Music with your Apple ID.
• Download music to your iPhone, iPad, or iPod touch in the Music app to listen to when you're offline.
• Turn on iCloud Music Library.
• Download past purchases that you made in the iTunes Store, App Store, or iBooks Store.
• Turn on Automatic Downloads for iTunes Store, App Store, or iBooks Store purchases.
• Turn on Family Sharing.
• Subscribe to iTunes Match.

Manage your account

You can manage your trusted phone numbers, trusted devices, and other account information from your Apple ID account page.

Keep your trusted phone numbers up to date

To use two-factor authentication, you need at least one trusted phone number on file where you can receive verification codes. You can update your trusted phone numbers when you follow these steps:

1. Go to your Apple ID account page.
2. Sign in with your Apple ID.
3. Go to the Security section and click Edit.

If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click next to the phone number you want to remove.

View and manage your trusted devices

You can view and manage a list of your trusted devices in the Devices section of your Apple ID account page.

1. Go to your Apple ID account page.
2. Sign in with your Apple ID.
3. Go to the Devices section.

The device list shows the devices that you're currently signed in to with your Apple ID. Select a device to view the model, serial number, and other useful information, including whether or not the device is trusted and can be used to receive Apple ID verification codes.

You can also remove a trusted device. Removing a trusted device will ensure that it can no longer display verification codes and that access to iCloud, and other Apple services on the device, is blocked until you sign in again with two-factor authentication. If you need to find or erase your device before you remove it from your trusted device list, you can use Find My iPhone.