I need to add boot via usb. I have Linux on usb flash drive.
Make sure your flash drive is UEFI bootable and if the system is running in IRST (Intel Rapid Storage/RAID) mode, that the distribution supports IRST.
Connect the drive and press F12 a few times at powerup. If the drive meets the above criteria, it will appear in the boot list. Select it, press enter and the system will boot from the flash drive.
If you do not see the drive in the boot list, it's not UEFI-bootable; you'll need to consult the documentation for your Linux distribution to find out how to create a UEFI-bootable drive.