Question: how to pxe boot a lenovo thinkpad?
Answer: Boot up the computer.
Press F2 -> press Enter -> press F1. ... 
Select Security -> select Secure Boot -> set to Disable -> select Start Up -> select UEFI/Legacy Boot -> set to Legacy Only -> press F10.
Reboot then press F12.
You should now be at the boot menu. ... 
Press Enter.