Payments to my 401k loan are being taken out of my post taxed income and should be pretax. What do I need to do to have this excluded from my taxable income on my W-2?

I work for a small employer and because of how my retirement account is managed, my loan is the first one the company has had to deal with.  The funds for payments are included in my taxable income on my paycheck and my direct deposit is split with my required payment going into a restricted account, the bookkeeper then takes my payment and mails it into the company that maintenances our plans.