Use temporary USB to install
memstick image. Then edit that image to have fixed IP address (or set up your DHCP server) and configure ssh connection (my recommendation is to use ssh keys, but not mandatory for temporary install).
Then boot from that USB stick on machine without monitor, ssh to it and install OS (
bsdinstall or extract
tar archivers or make installword).
I usually boot from SD/USB card or connect destination drive as USB and then manually do partitioning and OS install. My last adventure was on arm64 board on Linux with OpenZFS: create ZFS pool, create partitions, extract tars (base and kernel) into it, edit /boot/loader.conf and /etc/rc.conf, install bootloader (copy /boot/loader.efi to EFI partition), set u-boot/GRUB to load that file, reboot. Once logged set up network and ssh, setup
beadm, install rsync, rsync /usr/src and /usr/obj from x86 machine, install new customized world and kernel, reboot. All without monitor but with UART console (you even can skip that if you are sure that machine will boot).
Still works