Add Windows 10 to XPS Developer Edition(Ubuntu) Dual Boot

To my knowledge Dell doesn't have any official recommendations around setting up dual boot.  I know that Ubuntu is pretty good about setting up a dual boot solution if it finds an existing Windows installation when you start the Ubuntu installer, but I don't believe the opposite is true.  You might be better off looking around in more Linux-oriented forums for suggestions about adding Windows, but I suspect that instead of going through Windows Setup the normal way, you'd want to boot into it, then press Shift+F10 to open Command Prompt, and then use diskpart to create the necessary partitions and DISM to expand the Windows installation from the install media's WIM or ESD file onto that newly created partition.  The normal install process would also set up a Windows Recovery partition and the EFI boot partition, but the former is technically optional and the latter you might not even want to do if you're trying to maintain a dual boot solution, because a disk that contains one or more OSes is only supposed to have a single EFI boot partition, so Ubuntu would already have created one.  So after setting up the Windows image on your newly created partition, you'd probably have to go into your Linux boot loader and create an entry that points to it.  Again, this is all just a general idea.  You might find a more precise step-by-step on another more Linux-oriented forum.

After that, you'd need a Windows 10 license since one wouldn't have come with the PC.  If you don't already have one, you can buy one through the Microsoft Store or elsewhere.  And finally you'd probably want to download and install Dell Update, which will in turn download and install any necessary drivers.