How to add grass or trees to an imported plane from Blender 3D?
The plane is a .fbx

You actually can do this with a tool Unity team made for their Blacksmith trailer, its open source, they are just giving it within the blacksmith proyect files.
They call it PaintJob tool

This tool allows the artist to paint vegetation projected onto any geometry, not just Unity Terrains.

You can not add grass on a plane you have to use terrain for adding grass and trees