softwareupdate --fetch-full-installer --full-installer-version 12.6.5

note: this command to find your possible version at this point

--list-full-installers

Also, note the double dash ( -- ) is not what a copy and paste of the above post gives....

softwareupdate —fetch-full-installer —full-installer-version 13.3

so...

softwareupdate --fetch-full-installer--full-installer-version 13.3