public CIMStandardLabelPlacementProperties StandardLabelPlacementProperties {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.PointPlacementMethod = 
           StandardPointPlacementMethod.OnTopPoint;
else    //Current labeling engine is Maplex labeling engine            
  theLabelClass.MaplexLabelPlacementProperties.PointPlacementMethod = 
          MaplexPointPlacementMethod.CenteredOnPoint;

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