There are several methods of implementation.
First create serial number global field for each Associate. Then change your Case Number to be a text calculation of the id (12 or 14), the ampersand, a hyphen in quotes, an ampersand, and the serial number. Make this calculation NOT replacing the current value.
As the serial number increments with each new record you'll get a new number following the id and a hyphen.
There are major multi-user problem with relying on serial number fields, but they require complex solutions. If this works, don't sweat it. When your solution is used by multiple people, be prepared to change that serial number field for a REAL unique number generated possibly from a time stamp and a global of the current largest entry and or who knows what. That will PROBABLY work most of the time.

Here's the calculation for the Case Number field:
Table::AssociateIDfield & "-" & Table:: SerialNumberField
Happy fmp-ing!