You can use luci-app-vlmcsd to control it. luci-app-vlmscd support KMS auto-activation.
If you don't use luci-app-vlmcsd and you want vlmcsd support KMS auto activation, you should modify the settings of dnsmasq manually:
Add the following line at the end of /etc/dnsmasq.conf
:
srv-host=_vlmcs._tcp.lan,hostname.lan,1688,0,100
(replace "hostname.lan" with your actual host name, eg: openwrt.lan, or just replace it with your IP of LAN)
Restart dnsmasq:
/etc/init.d/dnsmasq restart
You can check if the dnsmasq setting works with the following cammand in Windows:
nslookup -type=srv _vlmcs._tcp.lan
The response should be your router's IP.
/etc/init.d/vlmcsd enable && /etc/init.d/vlmcsd start && /etc/init.d/dnsmasq restart
Your can find pre-compiled ipk: https://github.com/cokebar/openwrt-vlmcsd/tree/gh-pages