You do not get a tax deduction now, but the plan administrator will keep track of your basis in the account and this will reduce the amount of tax in the future. For example, if 10% of your contributions were made after tax, then 10% of your withdrawals when you retire will be exempt from tax. You can confirm this by calling the account custodian.
Note to future readers: contributions made outside of payroll deductions are not allowed for 401(k) and 403(b) plans, the 401(a) operates under different rules.
