Making button start external .exe file HELP!!!!

Soo...I dont want to open exe file inside game window, i want to make window from external file... And even if like i make something like this(For url because THERE IS NOTHING FOR OPENING FILES);

   using UnityEngine;
     using System.Collections;
     
     public class OpenInBrowser : MonoBehaviour {    
     
         void FFA() {
          Application.OpenURL("www.agar.io/?ip=192.168.0.1:911");    
         }
 
     
         void FFAFF() {
          Application.OpenURL("www.agar.io/?ip=192.168.0.1:912");    
         }
     }

when i make button in unity, and than go to inspector, i cannot add FFAFF(yea i made empty object...)

What can i do? PLEASE answer