I have modified one of the samples having an empty plane displayed and be able to move around. Now I want to display some polygons on the ground.
Right now I am only abe to add square rectangles to the scene by using another plane. But what if the polygon isn’t a square?
How can I do that?
Maybe you can get answer here Runtime Geometry Creation
Hello, and welcome to the forum! 
There are some informative topics related to lines,
Procedural Geometry Helpers,
A Mesh Generator …
There’s also DebugRenderer::AddPolyhedron (const Polyhedron &poly, const Color &color, bool depthTest=true) which I think should allow arbitrary polygons (you could just add one face).