Because IRA contributions are out of pocket (even if you set up direct deposit through payroll), payroll cannot treat them as pretax deductions. As far as they're concerned, it's just another bank account to deposit your paychecks**. However, this doesn't mean you can't enjoy the tax deduction now. To enjoy the tax deduction now, put the amount you plan to deduct annually on Form W-4 step 4b. This way, payroll will know to withhold less tax because your taxable income is lowered by the amount specified on step 4b.

**Strongly suggest that you do not set up direct deposit into an IRA, because any errors will cause more problems (e.g. excess contribution, reversed mistaken deposit being treated as early withdrawal).