You need to call EK to book the STPC. You will be provided with a voucher mini booklet and they should be able to have it ready for you at check in at BKK.

Since you're in Y, you can not chose a hotel, rather you will be put in EK Wing at the Airport Millenium Hotel which is a 2 min drive away from the arrivals. You will be provided with transfers (via bus which departs every 20/30 minutes). Also you will get at the hotel check in vouchers for a snack, breakfast and lunch (since your layover is this long).
Try and get the first floor 120 something rooms because they tend to be quiter and are smoke free.

There is an Emirates customer service desk in the hotel lobby which you could approach to request an emergency exit on your onward flight (they are usually pretty helpful, specially the folks that work late at night).