public CIMLabelClass[] LabelClasses {get; set;}

{
  // Note: the following should be embedded in a QueuedTask.Run() statement
  {
    //Get the layer's definition, using the community sample Data\Admin\AdminSample.aprx
    if (featureLayer.GetDefinition() is not CIMFeatureLayer lyrDefn)
    {
      // not a feature layer, leave
      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.
    featureLayer.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 visibility
    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
  }
}