Is there a way to know when the Build button was pressed? (the one in Build Settings).
I want to show a popup before the build starts.
I don’t think you can do it quite like that, but take a look at the build pipeline.
It’s quite easy to make your own panel to show in Unity’s UI with a “build” button, and make it do whatever you like. This also removes the need to navigate to the player settings when you want to make a build.
You can also perform post processing tasks if you need to.