Hello,
You should be able to go to the employee setup and go into their W4 setup and type in additional amount to withhold. Make sure their status and exemptions are all still correct and the only thing you're doing is adding the 15 dollars into the additional amount field.
Also keep in mind since you are updating W4, the system will probably force the NEW W4 form so make sure you know how the employee wants to claim according to the new W4 setup.
