I’m an Apple developer who has always registered devices by entering the required UDID in my Apple Developer’s account. The process to get your UDID has always been the same: hook up your device to iTunes, click on the device, and then click on the Serial Number in the device summary screen. The serial number changes to the UDID.
Since the release of the iPhone XS, XS Max, and the 2018 iPad Pro, this ID is no longer available in iTunes. I’ve looked everywhere and even contacted Apple support and no one knows where this ID is?!
Update and clarification: While it has come to my attention that this can be done in Xcode (and that's good for me), the client has nothing but Windows PCs and iTunes to accompany their iPad Pros. I need a solution that doesn't involve Xcode and will permit them to get me their UDIDs in a non-technical way.