how to access script not derived from monobehavior ?

[system]

namespace Prototype.NetworkLobby
{
public class LobbyManager : NetworkLobbyManager
{

           public void GoBackButton()
           {
                    backDelegate();
                    topPanel.isInGame = false;
           }
     }
 }

another script

 public class Escape : MonoBehaviour
 {
     void Update()
     {
         if (Input.GetKeyDown("f"))
         {
                // call GoBackButton(); from here
         }
     }
  }

[system]

Prototype.NetworkLobby.LobbyManager.s_Singleton.GoBackButton();