Open your administration guide and read about Extension Attributes, search EA in JAMFNation you should find hundreds of examples. You will have to learn some scripting to do this. JAMF also provides many templates in the EA module to get you started.

Are you referring to Extension Attributes? Those are the only custom fields I'm aware of, unless I'm confusing what you actually mean.
If you do mean Extension Attributes, that can be found under the Management Settings (or Computer Management) > Computer Management - Management Framework. Look for an Extension Attributes icon there.

Thanks much. I appreciate it.

If your JSS is tied to an LDAP server and you check 'Collect user and location information from LDAP' within the Computer Management -> Inventory Collection you then gain the ability to have 'LDAP Attribute Mapping' as an Extension Attribute input type. This allows you to pull in any value reported within a LDAP attribute. You could then make smart groups based off of the returned values.

I've also got some more information here, if it helps.

@mike.paul Ooo... good tip! Off to read up on that. TY!