[Yeti] Extrude on custom objects

Im new to parametric design and I’ve been playing around with Yeti. Could someone please take a look at this simple example and tell me what I’m doing wrong?
I’ve made 3 squares using a custom object “box” that takes a point and makes a square with that point as one of the corners. Now, I want to make a cuboid with that as the base. I tried following the extrude example but couldn’t figure it out.
Thanks in advance!
boxes.txt (506 Bytes)