I have an USB Ubuntu image which I have created previously and it worked.
I was not able to find any tutorials how to do it.
Configuring the computer to boot from the Ubuntu USB stick if it is inserted, otherwise to boot to Windows if it is not inserted can be done by changing the boot priority settings, which is often found under the Boot tab of the computer's BIOS/UEFI utility. The BIOS/UEFI utility can be accessed by pressing the key which is shown on the manufacturer's splash screen when booting. This is the F12 key on many computers. The keys to press, such as F12, F2, Del or Esc, differ on computers from different manufacturers.
When the power button is pressed to start the computer, it will try to boot first from whatever option is listed first in the boot priority section of the computer's BIOS/UEFI utility. If the USB stick is listed first in the boot priority list, it will try to boot from the USB stick first. If the internal hard drive where Windows is installed is listed first in the boot priority list, the computer will boot to Windows.
If the computer can't boot to the first device in the boot priority list because it is not attached, it will try to boot from the second device in the boot priority list instead. If the USB stick is listed first, but it is not currently inserted in the computer, the computer will skip the USB stick when booting and boot Windows from the hard drive instead, assuming that the Windows hard drive is the second device in the boot priority list.
Choosing Boot Drive Each Boot
You can choose the USB boot drive with every boot by pressing the appropriate Function key. With HP this is F9, my other computers use F10 or F12. You will likely discover this key looking for BIOS as suggested in other posts.
This may come in handy when booting your disk on a computer that is not yours. (Your friends may not like you messing with their BIOS).
Add USB Boot Drive to Grub
If you are using grub2 as a boot loader, You can easily add a Full install flash drive to the grub boot menu.
Plug in the USB drive and run sudo update-grub. The new entry will be shown in terminal as it is added.
If you make it the first boot entry in grub, the flash drive will boot first by default.
After you made your USB bootable with "Lili USB creator". Reboot your system until your motherboard welcome screen.
At the moment before the welcome screen pass, In most of the motherboards, you should press the "Del" button to load "Bios manager".
In the boot manager, "it usually exists in the setup tab or hardware" You will be able to see a list of boot hardware and it sorted with 1 or 2 or ...
You should set the "number 1 " every your default bootloader if you set a memory as number one when it's not plugged in, the system will use" number 2 " if the N2 is plugged.
It is up to you to how you sort them ...
