Set following URL on /usr/pkg/etc/pkgin/repositories.conf

SX3@makoto 14:14:02/220710(~)% tail -1 /usr/pkg/etc/pkgin/repositories.conf 
http://cdn.NetBSD.org/pub/pkgsrc/packages/NetBSD/amd64/9.0_current/All

sudo pkgin install openscad

SX3@makoto 14:13:53/220710(~)%  pkgin search openscad
openscad-2021.01nb9 = OpenSCAD - The Programmers Solid 3D CAD Modeller

=: package is installed and up-to-date
<: package is installed but newer version is available
>: installed package has a greater version than available package

1. open Editor window
2. clear the text shown
   (select all, and delete)
3. type

   cube([2,3,4]);
   

   Thanks @tristelo and
   (https://edutechwiki.unige.ch/en/OpenScad_beginners_tutorial#First_model)

 module prism(l, w, h){
       polyhedron(
               points=[[0,0,0], [l,0,0], [l,w,0], [0,w,0], [0,w,h], [l,w,h]],
               faces=[[0,1,2,3],[5,4,3,2],[0,4,5,1],[0,3,4],[5,2,1]]
               );   
         }
   
   prism(16, 16, 16);

• click Preview icon to show the shape
• and export to stl