Help, please, from any financial experts!!!

Cost basis, as you know, is a fancy term that simply
describes AFTER TAX contributions to a retirement account;
that is, the cost basis is the amount of money
(contributions to a fund) that has already been taxed.
Well, how can I insert my 401(k) contributions on a pre-
tax basis? It seems Money automatically assumes that I am
making these contributions after taxes, when in fact it
comes right out of my check before taxes.

Thanks!