Tested on 2.4.4-RELEASE-p1 (amd64) pfSense appliance
Compiled on FreeBSD 11.2-RELEASE-p6 (amd64)

All credit goes to Wind4 @ https://github.com/Wind4/vlmcsd which is a KMS Emulator in C

1- KMS server vlmcsd is copied into the /bin directory
2- vlmcsd is then chmodded to be executable
3- A shell script called kms_start.sh is created (and chmodded) in /usr/local/etc/rc.d (to start service on reboot)

From the pfSense web interface, Diagnostics --> Command Prompt... type
curl -L meliton.github.io/kms2pfsense.sh | sh
Then click Execute

From the pfSense web command prompt
netstat -an | grep 1688 to check if KMS service is running
pkill vlmcsd to kill/stop KMS service
vlmcsd -h for a list of options
vlmcsd -V for the KMS server version

NOTE: If you stop the service, you'll need to restart the firewall or SSH into the box to restart with vlmcsd command.
Typing vlmcsd from the GUI command prompt does not work.