That depends on how you are sharing the storage out of FreeNAS.
- NFS
-SMB
If you go the Freenas route as your backup target, you first have to setup the unit (network, users, etc.).
Create a pool and a dataset, which will be used as the Proxmox backup target. For instance, I have a dataset called "ProxmoxBackups".
You would than share that dataset through either NFS or SMB. In my case I have it shared through NFS.
Finally, you would add that NFS share within proxmox and select it as the vzdump backup location.