Does anyone know where I can modify the .exe game icon on Mac?

I'm on Mac and running Rocket League. I already have a custom Heroic game shortcut set in my dock with a custom icon, but when it launches the actual .exe it defaults to this icon. I would love to change this so that it shows with my custom icon in the dock, but I can't find where to replace this image (or if it's even possible) PS: Happy to share my custom Mac icon if someone can help me with this :)