Download FreeBSD memory stick image file (e. g. FreeBSD-13.0-CURRENT-amd64-20190531-r348447-memstick.img).

Copy the image file on an USB memory device. On macOS:

$ sudo dd if=FreeBSD-13.0-CURRENT-amd64-20190531-r348447-memstick.img of=/dev/disk3 bs=1m conv=sync

Connect serial cable to the host that needs the FreeBSD installation.

Use a tool like screen to connect to that host.

$ sudo screen /dev/ttyUSB0 115200

Start the host (e. g. connect power cable).

When FreeBSD boot menu appears, type 3 to enter loader prompt.

Set parameter and continue booting.

set comconsole_speed=115200
set console=comconsole
boot

Install FreeBSD as you like, but answer the last question before reboot (“The installation is now finished. Before exiting the installer, would you like to open a shell in the new system to make any final manual modifications?”) with yes.

Add these lines to the file /boot/loader.conf

comconsole_speed=115200
console=comconsole

Type exit to reboot in your new FreeBSD system.