The Y method for drawing boxes basically forces you to think about sets of parallel lines, and to infer vanishing points from the lines themselves, rather than having actual vanishing points present for you to refer back to. You're thinking about each box as being an independent entity, hence your struggle - but the fact of the matter is that the box that exists (the main body of the blender) and the box you wish to add to it (the button) already share all their vanishing points. So you're just adding additional lines to those existing sets - not introducing new sets. The boxes share their orientations in space, one is just a lot smaller than the other.

So, once you have the rectangle marked out along the surface of the existing box for where you'd like to extrude out the button, you just need to make sure the lines you draw coming out from that surface are converging consistently with one of the sets of the existing box.