The new version of FreeNAS 9.3.0 now can join Windows Domain Controller aka Active Directory easily. I was struggling to do this on previous version of FreeNAS but got no luck. I think the developer fix this issue and now we can easily join FreeNAS to existing Active Directory.
To join FreeNAS to existing Domain Controller, open the web management and go to Directory Service. Then click Active Directory.
Click enable and press Save. It will take some times. Wait until you see success notification on the top of the web management console. Now your FreeNAS is connected/joined with Windows Domain Controller.
Thanks for reading.