I have received a Known Traveler Number (KTN) from the TSA but do not know how to "activate" this pre-check authorization with various airlines. Is it possible to get this information into their systems when bookings for flights had preceded receipt of the KTN?
I realize this is an older question, but since none of the existing answers mention it, I just wanted to point out that you can indeed add your KTN on the airline's website for existing reservations, at least in most cases.
This blog post from TSA describes how to add a KTN to an itinerary on each of the airlines that are part of the Pre-Check program.
For example, on Delta (assuming you booked through them,) you can log in to your account on delta.com, go to "My Trips," and select the trip you want to update. You can then add your KTN in the "Secure Flight Passenger Data" section and click "Save." Note that this is not possible with 72 hours of the flight.
This is what the dialog looks like on Delta's website:
Usually it'll be on your ticket - a means to contact your carrier.
If not, calling is usually the most common option, but I prefer email as you get a written confirmation.
If you need a fast response, if they have a twitter account send them a tweet - most airline's social media guys are quick to respond, I've found.
It will vary by airline. In most cases, you will get the best results by calling the airline as you may not be able to change the secure flight information online after a reservation is ticketed.
I can say that American and Alaska Airlines helped me add my GOES ID number to pre-existing reservations once I was approved and it worked.