That is very hard to set the boot order in in XPS 13 Ultrabook. Even we tried to select the option boot from USB drive in the Windows Boot Manager. But we found the way.
Just following these steps:
1. Turn on the PC and press F2
2. set secured boot in the boot option menu to be disabled
3. change usb drive to be first in legacy list
4. Set the boot list to be legacy list
5. exit and save.
Then plug-in a bootable USB.