Your 401(k) contributions are shown in Box 12 of your W-2 with code "D", so you'll enter this information when you enter your W-2. (See the first FAQ below for more information.)
Note: If you took a distribution from your 401(k), you'll receive a Form 1099-R which you'll enter into TurboTax. (See the second FAQ below for instructions.)
Where do I enter my 401(k) or Thrift Savings Plan (TSP) contribution?
https://ttlc.intuit.com/replies/3302064Where do I enter my 1099-R? https://ttlc.intuit.com/replies/4194737