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Post-tax means the contribution amount is NOT subtracted from taxable gross, so you're simply setting up a new deduction code that is NOT listed in Payroll Exceptions for the applicable tax deduction codes.

Accurate setup can be confirmed using the following steps: 

1. Go to Deduction and Benefit Master. (Payroll>Payroll Setup>Deduction and Benefit Master)
2. Click Exceptions in the menu. Search for applicable tax deduction and a processing code of 2-No Deductions are Excepted Except. Click Accept. 
3. Verify the deduction being created is not listed in the Exclusion Ranges. 

When deductions are defined in Payroll Exceptions, it makes them Pre-Tax for the deduction code definedPre-tax means the contribution amount is subtracted from your taxable gross before you pay taxes on it.  Pre-tax deductions are listed in Payroll Exceptions with Processing Code 2 for all tax Deduction codes they should be subtracted from gross for.