Virtualmin isn’t your OS, you aren’t mountain anything inside Virtualmin.
NFS mounts are configured the same way on a Virtualmin system as on any other Linux system. You have a Rocky, so the RHEL docs are a reasonable place to start: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/managing_file_systems/mounting-nfs-shares_managing-file-systems
Webmin has a module for the fstab, and it also has a module for NFS exports (but that’s not the problem you need to solve, that’s for serving files via NFS, not mounting remote NFS filesystems…I just want to be complete in case anyone is searching for NFS in the future).