On my system, I can just run

$ sudo systemctl reboot --firmware-setup

And it’ll take the system to the BIOS.

From the systemctl(1) man page:

   --firmware-setup
       When used with the reboot, poweroff, or halt
       command, indicate to the system's firmware to
       reboot into the firmware setup interface for the
       next boot. Note that this functionality is not
       available on all systems.

       Added in version 220.

   --boot-loader-menu=timeout
       When used with the reboot, poweroff, or halt
       command, indicate to the system's boot loader to
       show the boot loader menu on the following boot.
       Takes a time value as parameter — indicating the
       menu timeout. Pass zero in order to disable the
       menu timeout. Note that not all boot loaders
       support this functionality.

       Added in version 242.