On the NAS create a shared folder you want to mount and a user with the rights for the shared folder
Go to the shell of proxmox
nano/etc/fstab
add this line and updated to the correct values form the NAS //192.168.178.41/PlexMedia /mnt/PlexMedia cifs username=Plex,password=1u1GX7ZlBQ5kJej9ZAG0U+,iocharset=utf8,noperm 0 0 to the file and safe and exit now it works importent check that the password has no special charecters because this can make a problem like / etc
Now you need to create the folder on proxmox you want to mount to like here mnt/PlexMedia so create the PlexMedia folder in the mnt directory
To mount the folder you need to run this command to connect with the NAS and the folder should be mounted correctly