From MainActivity to MapFragment

Hello

Coming from C#…
I’ve created a Navigation drawer app and added a MapFragment.
Now how can I reference from the MainActivity to MapsFragement?

I want to set a marker as soon as I press on my FloatingActionButton

val fab: FloatingActionButton = findViewById(R.id.fab)
fab.setOnClickListener { view ->
val swiss = LatLng(47.0,9.0)
map.addMarker(MarkerOptions().position(swiss).title(“Swiss”))
}

map cannot be referenced…

Thanks