Find the iPhone's UDID

On the iPhone that will get the app, open udid.tech in Safari and tap Get My UDID. Allow the download, open Settings → Profile Downloaded → Install. Safari returns with the UDID; copy it.

The site installs a small configuration profile that reads the device id and removes itself; it says it stores no data. The UDID is the phone's serial-like id, not a secret. You can remove the profile under Settings → General → VPN & Device Management.