It's there, it's just not obvious.
Go to Manage your itinerary and log in via your Confirmation number and passenger name. After you log in, you'll see a box on the upper left. Click on the APIS link:




The next page is long. Scroll down a bit, and you'll see where you can add the KTN.



It'll only be useful for boardings in the USA. Not so much on the way back (unless you're connecting to somewhere else after transiting Immigration/Customs PoE to a domestic USA segment).

Edit: Also, I recommend doing it as soon as possible. Most of the time, entering it at check-in should work, but there's a non-trivial chance it won't, depending on the timing of check-in, especially if it's near cut-off. Entering it now puts your mom in the bulk data transfer the airline does the day before.