You should receive a Form 1099-R from your 401-K brokerage that shows both the distribution and the taxes withheld. However, the deadline to send out those forms is January 31, 2017. If you don't have it by then, I would contact your brokerage.
You will then report the information from that Form 1099-R in TurboTax using the directions in the FAQ below:
https://ttlc.intuit.com/replies/4194737
