As a system administrator I can tell you that system keys are not given out. It's like handing a complete stranger the key to your front door and saying "Come on in, put your feet up and help yourself to anything in the fridge!". If you are a legal user of the system, the system administrator will set up the radio for you. If you are not a legal user of the system, they will not, and they will not give you a system key. I keep my system keys locked up and no one gets hold of them that isn't directly authorized by me.
Some people have hacked system keys, and have made it work, but that doesn't make it legal.
Incorrectly putting a radio on a trunked system like that can cause issues. Done wrong, it can take resources from public safety users. Using someone else's radio ID can make their radio not work. Done wrong, it'll show up to the system admin, and they'll brick your radio. You can blame it on the radio tech who did the programming, but if you are the one holding the radio, they'll take you down also.
I mean this in the most helpful way: Get a scanner. Easier, legal and will do way more than a old Motorola radio will.