Showing wireframe in a primitive

Hi there.

This is such a simple situation, but I’ve been searching for about two hours and I can’t find any information about this.
I’m trying to show the primitives (triangles) within a rectangle as a wireframe in order to work on a custom grid mapping inside a primitive. The thing is, I did see it happen in the documentation ( Custom Geometry & Appearances |
I just can’t find any information or parameter that allows me to achieve this result.
Thanks for the help, hope it’s actually doable!