over 10 years ago I used plugin as OpenVPNSSPI.dll and it works great you can try it , just down load openvpnsspi.dll and copy it to openvpn folder and open config file add this line to it
plugin "C:\\Program Files (x86)\\OpenVPN\\bin\\OpenVPNSSPI.dll"
https://code.google.com/archive/p/openvpnauthwindowsplugin/
if you have more time you can make other authencation wonk on domain controller just go to internet type amigo4life2 and you can file how to config domain controller authencation , iI did it over long time ago and it work very fast , i can help you if you like to config it , just try it yourself first