Setting up passwordless login on Linux is pretty easy to do. In order to complete this you need to add your user to the ssh group. This can be done in the User section of the Access Rights Management.
If you have the users home directory setup this is the way I have done it with user /bin/bash. This can be changed in the GUI for the user in the User section under Access Rights Management.
Generate a new key(you can also use an existing key if you have one already)
Whatever you called it keep that in mind. Now we are going to copy it to the user account.
That should be it. To test your login with your key instead of a password do.
If you can login successfully I recommend disabling password login and root login after you have successfully tested this.
Also a good idea to setup Fail2Ban if your NAS is exposed to the internet.