// OnClick action for additional toolbar button
public override bool OnOptionsItemSelected(IMenuItem item)
{
  switch (item.ItemId)
  {
    case Resource.Id.editItem:
      OnEditItem_Clicked();
      return true;
    default:
      return base.OnOptionsItemSelected(item);
  }
}

...

// Responsible for inflating additional toolbar menu
public override bool OnCreateOptionsMenu(IMenu menu)
{
  MenuInflater.Inflate(Resource.Menu.helper_nav, menu);
  return base.OnCreateOptionsMenu(menu);
}