how to add device (Ipad)

To add a device to your AppleID account, simply set-up the device and sign-in with your AppleID and password.

Set up your iPhone, iPad or iPod touch – Apple Support

You can check which devices are associated with your account here:

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

See this support article:--> Check your Apple ID device list to see where you're signed in - Apple Support

(section "Add a device to the list")

Regards

Giulio