How to set up a goal in Yandex.Metrica for a pop-up window?

Question

JavaScript

I want to track pop-up windows (error messages). After clicking on the button, an error message may appear - this is what I want to track.
Does this require a compound goal?
For a button would be: onclick="ym(XXXXXX, 'reachGoal', 'btn-next'); return true;"
And what about the popup?