How do i Achieve mesh Extrusion?

Have you tried studying the source code of https://www.assetstore.unity3d.com/en/#!/content/5141 ?

Here you have a simple example, this code is added to a Unity plane primitive mesh:

 srcMesh = (GetComponent < MeshFilter> ()).sharedMesh;
          precomEdges = MeshExtrusion.BuildManifoldEdges (srcMesh);
         
         Matrix4x4[] sections = new Matrix4x4[3];
         
         sections[0]=transform.worldToLocalMatrix* Matrix4x4.TRS(transform.position,Quaternion.identity,Vector3.one);
         sections[1]=transform.worldToLocalMatrix*Matrix4x4.TRS (transform.position+4f*Vector3.up, Quaternion.identity, Vector3.one);
         sections[2]=transform.worldToLocalMatrix*Matrix4x4.TRS (transform.position+6f*Vector3.up, Quaternion.identity, Vector3.one);
         
         MeshExtrusion.ExtrudeMesh (srcMesh, (GetComponent < MeshFilter> ()).mesh, sections, precomEdges, false);
     
 This code simply elevates the plane (creates a box), with two sections. You have use the transformation matrix (Matrix4x4) to tell the script where your sections end, so first you get a transformation matrix to convert your world position to local coordinates (transform.worldToLocalMatrix), then you apply (by multiplying) another transformation  to tell where you want to end up from your local point of view (Matrix4x4.TRS (transform.position+4f*Vector3.up, Quaternion.identity, Vector3.one)).