Because we're doing more estimation in this exercise than the last one, it's very common for students to... well, panic. And when they panic, they don't think. So if you catch yourself panicking, take a step back and go back through these instructions.
As we're using one concrete vanishing point for all the boxes in the scene, it means that all of these boxes are aligned in a very specific fashion. Their faces are not slanted at all relative to us or the ground plane.
What this ultimately means is that for this step, all we need to do is draw a rectangle. Do this using the ghosting method - meaning, plot all 4 points, focusing primarily on keeping them aligned with one another. Keep in mind we're going for a rectangle here, not a square, so your proportions don't matter.
With your points plotted out, go through the preparation and execution phases of the ghosting method to connect them with confident edges.
Remember that the ghosting method is all about breaking the process into steps so your final execution can focus on being confident, rather than worrying about accuracy. This may of course lead to your lines going a little off track, but that is of course entirely okay, and simply something that you'll get better at with practice.
As you can see here, I'm making plenty of little mistakes, as I'm admittedly rusty from spending most of my time teaching, rather than drawing.