Just click on the box beside Ready Or Not, then scroll down and fill in your name and confirmation code only!
If you want you can choose two other flop songs along with it so it doesn't seem like fan-requesting but i don't think it matters a lot.
This one played her but hasn't added her to the list yet, that means you can write it in the box where it says "Don't see your song above? Type it in here!" (just type Bridgit Mendler - Ready Or Not) at the bottom.