You mean on the file in unity that name is: secondary scene, i put my own name?
On your" MainScene" there is an object called GameHandler or GameManager if you select it and watch the details panel it says win game load scene "WinScene", lose game load scene "LoseScene", you just need to put there the name of your secondary scene. for example if you have a scene called "SecondaryScene" put that. Rember to add the scene to the build