tested with thinkpad and openbsd 7.8

update thinkpad boot screen

prepare bootable drive

plug in usb drive as sd2
download and run boot.sh

warning! all data on sd2 will be erased.

# ftp -o - https://romanzolotarev.com/tp/boot.sh |
  sh -s sd2
#

boot.sh downloads an official bios update image for your thinkpad (for example: x1c5 or t14g3), installs geteltorito to convert .iso image to .img, writes it to the usb drive, and copies logo.gif.

warning! next step may brick your thinkpad.

boot usb and follow prompts.