public bool PlaceOnlyInsidePolygon {get; set;}

{
  // Note: call within QueuedTask.Run()
  {
    //Get the layer's definition
    var lyrDefn = featureLayer.GetDefinition() as CIMFeatureLayer;
    //Get the label classes - we need the first one
    var listLabelClasses = lyrDefn.LabelClasses.ToList();
    var theLabelClass = listLabelClasses.FirstOrDefault();
    //Modify label Placement 
    //Check if the label engine is Maplex or standard.
    CIMGeneralPlacementProperties labelEngine = MapView.Active.Map.GetDefinition().GeneralPlacementProperties;
    if (labelEngine is CIMStandardGeneralPlacementProperties)
    {
      //Current labeling engine is Standard Labeling engine
      theLabelClass.StandardLabelPlacementProperties.PolygonPlacementMethod =
               StandardPolygonPlacementMethod.AlwaysHorizontal;
      theLabelClass.StandardLabelPlacementProperties.PlaceOnlyInsidePolygon = true;
    }
    else
    {
      //Current labeling engine is Maplex labeling engine
      theLabelClass.MaplexLabelPlacementProperties.PolygonFeatureType =
                               MaplexPolygonFeatureType.LandParcel;
      theLabelClass.MaplexLabelPlacementProperties.AvoidPolygonHoles = true;
      theLabelClass.MaplexLabelPlacementProperties.PolygonPlacementMethod =
                          MaplexPolygonPlacementMethod.HorizontalInPolygon;
      theLabelClass.MaplexLabelPlacementProperties.CanPlaceLabelOutsidePolygon = true;
    }

    lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
    featureLayer.SetDefinition(lyrDefn); //set the layer's definition
                                         //set the label's visiblity
    featureLayer.SetLabelVisibility(true);
  }
}