apt install nfs-kernel-server
cd /usr/share
mkdir nfs-shared
chmod 757 nfs-shared
echo "/usr/share/nfs-shared/ <IP_client>(rw,root_squash)" >> /etc/exports
service nfs-kernel-server reload

Go to Datacenter → Storage→ Add → NFS

• ID: nfsproxmox
• Server: <IP_NFS_server>
• Export: /usr/share/nfs-shared
• Content: what you want
• Nodes: what you want
• Enable : check
• Max Backups: 1
• Add

You can now use the shared NFS filesystem.

If you want to allow other clients to use nfs sharing, add /usr/share/nfs-shared/ <IP_other_client>(rw,root_squash) in /etc/exports.