The idea behind low poly mesh is to duplicate vertices to achieve sharp edges. When we normally create a mesh, it has N vertices. And when you recalculate normals, Unity generates exactly N normals. That means that for each angle in your mesh there is only one normal vector, which 3D engines try to smooth. So supposing a simplified 2D example, it looks something like this:

Your goal instead is to duplicate vertex for each triangle, that this vertex belongs, so that each edge would be disjoint, and that would cause Unity to generate separate normals for each edge and sharp the angle. Like this:

Unfortunately, there is no way to create multiple normals for one vertex, that's because you'd have to duplicate them. So for example you want to create a mesh with two triangles with a sharp edge. In code it would be something similar to this:

var mesh = new Mesh();

// Basically amount of your vertices is equal to amount of indices.
var vertices = new Vector3[6];
var triangles = new int[] { 0, 1, 2, 3, 4, 5 };

// Define first triangle.
vertices[0] = new Vector3(1, 0, 0);
vertices[1] = new Vector3(0, -1, 0);
vertices[2] = new Vector3(0, 0, 0);

// Define second triangle
vertices[3] = vertices[2];    // Duplicating vertex #2.
vertices[4] = vertices[1];    // Duplicating vertex #1.
vertices[5] = new Vector3(-1, 0, 1);

mesh.vertices = vertices;
mesh.triangles = triangles;
mesh.RecalculateNormals();

Also check this article, it describes nicely behaviour of shading in 3D engines depending on shared and separated vertices: Tutorial 9: VBO Indexing

Update: As mentioned in the comments you can also try to use Flat shading model. But as far as I know, it might be not supported on some platforms.