How would I enter an amount in box 32 Travel in a prescribed zone

Box 32 NTA has already been taxed b/c the amount to be designated to Box 32 is determined from the gross pay total at the end of the year.  This amount should not be added to gross income, but be a result of income  earned in a northern area.  It is not an amount for the company, but an amount for the employee.  If I set it up as a benefit it is added to Gross Income.  

I think it should be entered as a comment only - on each t4, and each employee is different.  Any ideas how I can do this?