public CIMLabelClass[] LabelClasses {get; set;}

var lyr = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault(f => f.ShapeType == esriGeometryType.esriGeometryPolygon);
if (lyr == null) return;
QueuedTask.Run(() =>
{
  //Get the layer's definition
  //community sample Data\Admin\AdminSample.aprx
  var lyrDefn = lyr.GetDefinition() as CIMFeatureLayer;
  if (lyrDefn == null) return;
  //Get the label classes - we need the first one
  var listLabelClasses = lyrDefn.LabelClasses.ToList();
  var theLabelClass = listLabelClasses.FirstOrDefault();
  //set the label class Expression to use the Arcade expression
  theLabelClass.Expression = "return $feature.STATE_NAME + TextFormatting.NewLine + $feature.POP2000;";
  //Set the label definition back to the layer.
  lyr.SetDefinition(lyrDefn);
});

//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();
//Set the label classes' symbol to the custom text symbol
//Refer to the ProSnippets-TextSymbols wiki page for help with creating custom text symbols.
//Example: var textSymbol = await CreateTextSymbolWithHaloAsync();
theLabelClass.TextSymbol.Symbol = textSymbol;
lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition
                                     //set the label's visiblity
featureLayer.SetLabelVisibility(true);

//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