Okay, you ready for this? I'm gonna give y'all the low down on how buttons work in Amnesia. Let's gooo...
Go into your level editor and select the button you want to be press-able. From there go to Entities and go down to the box that says PlayerInteractCallback (d'ya see it?). Okay, type whatever you want into this box (let's say PressButton in this case). Now go and take your already developed script and make it look a little like this:
That's right, we don't need to have something in the OnStart for this, it's all LevelEditor & callback, baby.
Okay, hope that helped. See ya.