Hello, and welcome.
When you applied for and received your KTN was it for TSA's Precheck program or another program?
I was hoping to hear back from you, but I can send my answer now, and if you have any further questions, just hit 'Reply.'
When you book your flight, you need to tell the reservation agent to include your KTN on the ticket and boarding pass, in order for the chance to receive expedited screening, when you have TSA Pre (Precheck). However, please be aware that even if you have a KTN, you are not necessarily guaranteed expedited screening on every flight. The TSA's system chooses passengers randomly for each flight, to receive expedited screening, in order to add extra levels of security.
Since your flight is already booked, and this was done before you received your KTN, plus, it's less than a week away, you can call your airline and ask if you can add your KTN now, and get it printed on your ticket and boarding pass. They could say 'no', that it's too late, or they might be able to do it for you.
The next time you book a flight, first of all, you can create a 'profile' with each airline you use and include your KTN on there, so they always have it and when you make your reservation, ask them to include the number on your ticket/boarding pass.
I hope this helped you and if so, please kindly let me know, here and/or Rate my answer positively, as this is the only way I am recognized for my assistance. Thank you very much!
Again, for any additional questions, even after rating, just click 'Reply' and I will be glad to continue to help.