How to interact with MKMapView subview in UI Test

When I normally write UI Tests, I query for a button by using

app.buttons["accessibilityIdentifier"].firstMatch

However, when I want to do the same for a similar button that is on screen, I cannot get it to work. This button is inside of a simple UIView container that is added to an MKMapView by using

map.addSubView(popupWithButtonView)

How would I interact with this buton in my UI Test?

I have tried the following, without success:

app.buttons["trueButton"].firstMatch
app.maps.element.buttons["trueButton"].firstMatch
app.maps.buttons["trueButton"].firstMatch

The accessibilityIdentifier is set correctly, as when the popup normally occurs when debugging, I can see it with the 'Debug View Hiearchy' tool.

Any idea what the right way is to query for this button?