You will need to create an SSH key on another system/SSH client software. Once the key is generated, it would be easiest to cut and paste the key into the user's SSH key location via the PortServer's Web UI. The tftp method can also be used to upload the SSH key however, pasting it into the Web configuration is easiest.
Once the SSH key is pasted in the PortServer user entry, you can connect using your SSH client software.