Excellent, then do please carry on as if I didn't say anything.
The System Key is an authorization key carried by each radio that gives it the appropriate permissions to affiliate to a system. That key has to be added to the radio by the system administrator. Typically, unfortunately, Advanced System Keys are the most common type these days, and they are usually given a limited number of radios they can be written to by the manufacturer, based on the capacity of system the administrator has purchased.
Basically TLDR, the System ID just says which system it is. The System Key provides the appropriate permissions for the radio to use the system and be authorized by the system. System Keys are generally fairly tightly controlled both by administrators and the manufacturer. Best thing, get in touch with the owner and see if you can get the system key added to your radio. Once it's in there, you should be able to take care of the rest.