Since the contributions are pre-tax they amount would be deducted from your Box 1 wages on your W-2. You do not need to enter the information any where else in TurboTax.
The amount you contributed is reported in Box 12 of your W-2 and if you look at Box 1 and Box 3 and Box 5 the amount in Box 1 is less than the amount in Box 3 and 5. This is the amount you contributed to the plan.
Frequently Asked Questions about PERSI