Sorry, first time flying KE, so maybe a noob question.

I was trying to book a flight from houston to seoul on Korean Air's US site, and want to credit the flight to my Alaska mileage plan account. But I couldn't find any place to put my frequent flyer number on there. During the process, I was only able to put Korean Air's Skypass number, but no where could I put another airline's frequent flyer numbers.

Was I doing something wrong? Please help

Thank you