Answer by Airship Games

It was actually easy. After a bit more digging, I found the solution:

using UnityEngine;
using System.Collections;
public class OFAchievement : MonoBehaviour {
private static OpenFeintFacade openFeint;
void Start(){
    openFeint = new OpenFeintFacade();
    openFeint.UnlockAchievement(941962);
}
}

Now, when the mission starts, the Achievement 941962 is unlocked.