Hi,
"Server secret" seems to be a settings specific on NetApp filer so you could also try to contact NetApp for further information.
I searched on NetApp website and BrachCache is supported in newer versions. So they should help an instruction about "how to setup".
This is a thread I found on NetApp website:
https://communities.netapp.com/thread/24010
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
If you have any feedback on our support, please send to tnfsl@microsoft.com.