Created a new account just for this.
In case anyone wanted to know how to add their KTN to their Icelandair reservation and they didn't want to call...because I saw folks having issues finding it (and Icelandair Customer Support people having that issue finding it too as of 2022!)...go to 'Manage booking' and click on 'Edit contact information'. There you can add your passport, visa, emergency contact information, and yes, even your KTN. Remember to put the issuing country in the drop box when putting in your KTN, otherwise it'll give you an error if you try to 'Continue'/'Save Changes'.
You're welcome!