Create a new game object/prefab in a scene using C# code for Unity.

add-prefab-game-object-to-scene.cs

using UnityEngine;
using System.Collections;

// I attached this script to my main camera
public class GenerateMap : MonoBehaviour {
  void Start () {
    int x = 0;
    int y = 0;

    // Adding a Prefab/GameObject to Scene using C#. Make sure you create a prefab with the file name
    // "platform-lg" and put it in /assets/resources/

    //GameObject cube = GameObject.CreatePrimitive(PrimitiveType.Cube); // Create basic cube instead of prefab
    GameObject cube = (GameObject)Instantiate(Resources.Load("platform-lg"));
    cube.AddComponent<Collider2D>();
    // cube.AddComponent<Rigidbody>(); // add rigid body
    cube.transform.position = new Vector3(x, y, 0);
  }
}