Set value on add to scene

// Assets/Editor/TestEditor.cs
using UnityEngine;
using System.Collections;
using UnityEditor;

[CustomEditor(typeof(Test))]
public class TestEditor : Editor {

    private void Reset()
    {
        Debug.Log("Reset");
        Test test = (Test)target;
        test.SetVal(5);
    }
}


// Assets/Test.cs
using UnityEngine;
using System.Collections;

public class Test : MonoBehaviour
{
    [SerializeField]
    private int myVal;

    public void SetVal(int val)
    {
        myVal = val;
    }

    private void Start()
    {
        Debug.Log("myVal: " + myVal);
    }
}