You can setup a garnishment  field on the User-Defined Fields screen.  Open the Payer in AMS Payroll.  At the AMS Payroll Menu, select Payer>Payer Edit>User-Defined Fields.  Double-click on the first blank line.  Enter in a field name (for example, Garnish) and select the appropriate settings.  You can also select Examples from the Edit screen to view pre-set User-Defined Fields screen.