4:43 PM, Sunday July 18th 2021
Hi!
That is a one point perspective and there is only one vanishing point for the scene, the one you have already established. Lines that do not go toward the vanishing point are either perpendicular or parallel to the horizon line, as you have already drawn them.
I have completed the box with color coded lines and notes: https://imgur.com/a/j4Jz63v
I would always think in terms of lines that go toward the VP, that are parallel or perpendicular, leaving the right angle thing as a final check to see if the front and back planes are drawn correctly.
EDIT: oh wait a moment, maybe I have misunderstood your question: did you want to extrude the box to the left? If that is the case I'll do another picture.