Happy New 2019 Year!
Download the CLI guide for the Sx300 switches:
page 24 - User (Privilege) Levels
Hope it helps.
here is the configuration from GUI
yes i support other post you can use radius/tacacs/acs - can do this job for you, since this SMB and only 1 router not make any sense for business case to have Radius server.
but options available many if you can able to install another component for this task.