Originally Posted by Maxwell Smart

Below is pasted from the TK website. The bolding is mine:

TSA Precheck® helps you to save time at security checkpoints, making Turkish Airlines' USA flights more practical.

With TSA Precheck®:
[List of Pre-check benefits removed to save space]

In order to experience TSA Pre✓®, members enrolled in a trusted traveler program must add their Known Traveler Number to their reservations with Turkish Airlines at the kiosks or check-in counters for the flights departing from below listed airports;

Boston (BOS)
Dallas (DFW)
Houston (IAH)
New York (JFK)
Orlando (MCO)
San Francisco (SFO)
Seattle (SEA)
Chicago (ORD)
Washington (IAD)

Dear Passengers,

Go to our check-in counters or kiosks and enter your Known Traveler Number into your saved or newly created reservation. If you have forgotten your Known Traveler Number, please contact our call center or visit our check-in counters or kiosks at the airport.

If you continue your journey with another airline, enter your Known Traveler Number for all flight segments to enable your TSA pre-check® to be valid for your onward flights.