Mobile Passport Control (MPC APP); Adding Known Traveler Number?

I am trying to add my TSA Precheck number under "known traveler number" on the MPC (Mobile Passport Control) app on my android (Samsung) phone. It seems to only allow for numbers while the TSA Precheck also contains letters. I even tried cutting and pasting it in and it caused random numbers to appear. Anyone else encounter this? What did you do?