Quiz Scoreboard

Hello, I want to make a script for score, at each question answered right the player gets a point but I can’t figure how to make the script, so here it is:

using UnityEngine;
using System.Collections;

public class Final : MonoBehaviour {

	public GUIStyle myStyle;
	public int Score=0;

	// Use this for initialization
	void Start () {

	}
	// Update is called once per frame
	void Update () {
		if(Application.ExternalCall("RightQ1"))
			Score+= 1;
		}

	void OnGUI(){
		if(Score > 0)
			GUI.Box(new Rect(0,0,Screen.width,Screen.height)," ",myStyle);
	}

	void Awake(){
		DontDestroyOnLoad(transform.gameObject);
	}
}

And I get this error: Assets/Scripts/Final.cs(16,17): error CS0029: Cannot implicitly convert type void' to bool’
What am I doing wrong?

Assets/Scripts/Final.cs(16,17): error
CS0029: Cannot implicitly convert type
void’ tobool’

The error is line 16:

    if(Application.ExternalCall("RightQ1"))

The return type of that function is void:

You cannot use an if statement with a void function call.

Then how do I make it work?