Looks like the bracket they give has a bit of depth to it, and I take it that ends up inside the wall
Cut a square of plasterboard that will be hidden behind the tv, out from between two adjacent studs
Cut a square of hardwood 18mm ply at least 3 times the height of the bracket and as wide as the gap between studs
Glue and screw battens to the edge of the ply face, on the sides that touch the studs
Mount the ply by screwing through the battens sideways into the studs, such that the ply is deep inside the wall and the perfect depth that the TV will be flush with the wall when on the bracket
Hang the TV hiding the hole in the wall. Use the internals of the wall as a service void for cables because nothing ruins a flush look faster that an hdmi lead snaking its way up from the DVR below to behind the TV