This short tutorial by user randomproblem shows us how to get FreeBSD installed via serial port connection. Follow the link below for the full instructions.

I recently wanted to install FreeBSD on a PC Engines APU device. The APU is a headless device, so it is not possible to connect a monitor. However, it does have a serial port.

For some reason, the current FreeBSD 10.3 documentation appears to be missing the information on headless installation that was present in the 9.3 documentation. (In fact, the entire section on advanced installation topics is missing.)

Fortunately, the procedure that was described for 9.3 still works with 10.2 and 10.3:

  1. Create an installation USB stick as described in the handbook.

  2. Mount the medium on a running FreeBSD system: …

Full tutorial: https://randomproblem.wordpress.com/2016/05/15/installing-freebsd-via-a-serial-port/