pkg install vm-bhyve
pkg install vm-bhyve-firmware

openbsd.conf

loader="uefi"
cpu=4
memory=4G
network0_type="virtio-net"
network0_switch="public"
disk0_type="virtio-blk"
disk0_name="disk0.img"
disk0_size="40G"
bhyve_options=""
uuid=""
network0_mac=""

settings.conf

console="tmux"

• The above is a default openbsd.conf template

• And the bottom is to use the tmux as console backend instead of cu which I personally don't like.

when you are at the boot prompt

Using drive 0, partition 3.
Loading......
probing: pc0 com0 com1 mem[638K 1918M a20=on]
disk: hd0+ hd1+
>> OpenBSD/amd64 BOOT 3.33
boot> set tty com0

set tty com0

when you see the boot prompt press space or any key and type in

set tty com0 and press enter you will see a different prompt as you are now using the serial console.

and the OpenBSD should boot and a menu should appear.