Bringing window to back GUI.BringWindowToBack()

I'm fairly new to coding and I am trying to get my main window(Game mode select) to go to the back, or as an alternative how can I put a GUI.Box withing another GUI.Box. I found a link a while back and happened to forget :/ ERROR: Assets/Gui.js(6,17): BCE0023: No appropriate version of 'UnityEngine.GUI.Box' for the argument list '(int, UnityEngine.Rect, String, UnityEngine.GUIStyle)' was found.

 function OnGUI () {
 
     // Make a background box
     GUI.Box (15,Rect (0,0,Screen.width,Screen.height), "Game Mode Select", customButtonBackground);
 
     GUI.BringWindowToBack(15);
       
     GUI.Label (Rect (25, 25, 100, 30), "Game Menu");
     
 }