iPhone XS doesn't have UDID
- Go to "->About This Mac".
- Click on "System Report" and select "USB".
- Find your phone and the "Serial Number" field is what you are after.
- Copy this value and paste it into the developer portal when you register a new device; you will need to insert a - after the 8th digit.
You can also get the "identifier" from the Xcode "devices" window or have Xcode update the portal directly.
You can compile the UDID from the ECID that is shown by iTunes with the following formula (for XS and XS Max)
The CHIP version in the XS/XS Max case is 8020 (source for it)
So if you want to compute the UDID for an XS/XS Max (and I believe the XR has the same chip), then you'll need to replace the ECID with your own in:
To find the ECID, connect the iPhone to iTunes and click the serial number, the ECID will show up.
Edit: For the XR the formula is: 00008020-000ECID, because the ECID has one less character