If you read this post:
You will need access to the command-line: https://osmc.tv/wiki/general/accessing-the-command-line.
Its already been enabled in the kernel, you just need to update /home/osmc/.smb/smb.conf; as discussed in the thread. Then reboot.
Edit: Thought of this after I finished posting:
You initially probably only want to add to add:
client max protocol = SMB3
to smb.conf, so you can browse and add your shares. Adding the other options mentioned in thread, breaks browsing but is more secure.