ButtonText.cs

// attached this script to root game object of 'Button Prefab' with UIButton(NGUI).

using UnityEngine;
using System.Collections;

public class ButtonText : MonoBehaviour {
	[SerializeField] UILabel label;
	public string text
	{
		set {
			label.text = value;
		}
		get {
			return label.text;
		}
	}
}

TestButton.cs

// attached this script to 'Button1'/'Button2' game object of 'Window Prefab' with PrefabInPrefab.

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class TestButton : MonoBehaviour {
	[SerializeField] string text;
	[SerializeField] string message;

	void Start()
	{
		var buttonText = GetComponent<ButtonText>();
		if(buttonText != null) GetComponent<ButtonText>().text = text;
	}

	void OnClick()
	{
		Debug.Log(message);
	}
}