Hi, Melinda,

It sounds like your need would be met in Employee Maintenance>Wages>Tax Status, where you would choose "Add Amount" from the "Override W/H Tax Calc" dropdown. Enter $100 in the "Override Amount" box and save the change. 

I had been over and over the Employee Maintenance trying to find a way to do this as I knew it must be available. I am using 100c Advanced 2018, so I don't have a Wages tab, but buried deep in the Taxes tab I found the Tax Calc Override once I knew what I was looking for, and was able to add the $100. Thank you for your help! Relying on my memory to take care of it every time is not a reliable method. 

Yes it is to the far right on the Taxes tab. You can move these fields where you want them.