I've gotten online instructions and the above piece is required, but annoyingly the piece ID is not in the instructions and I can't find it on any lego websites. I just want to know what the piece ID is.
This is the ID 4211760 which in the LEGO movie sets holds the piece of resistance in place on Emmet's back. See for example instructions of set 70801 (Melting Room).
Bricklink lists it as "Minifig, Neck Bracket with Back Stud" (42446).
Also you can use app for Android (or iPhone) for finding necessary piece. https://play.google.com/store/apps/details?id=com.betab.partsguideforlegotechnic