External Event Handlers

I'm dealing with code.org's AppLab for one of my projects. I'm using Scala and compiling to Scala.JS.

This Scala registers event handlers to AppLab's onEvent function but prepack is recognizing it as dead code.

How do I properly handle/configure this case? (There are other functions that mutate external state as well, like button, setPosition, setText that I use in my code, but I handle these with the function described in #619)