How to add an Apple iPad Air to my Apple ID?

Sign in on your device

Read this support article to learn how to sign in with your Apple ID on various kinds of 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.

On the iPad Air, go to Settings > [your name] > Find My > Find My iPad and turn on Find My iPad. I believe it should be sufficient for the iPad to be listed in the list of devices.