public string ArcadeExpression {get; set;}

var usaStatesLayer = map.GetLayersAsFlattenedList().OfType<FeatureLayer>()
                 .FirstOrDefault(fl => fl.Name == "USA States");
  if (usaStatesLayer == null) return;

  //Note: Run within a QueuedTask
  //Create a color ramp from red to blue
  var colorRampForPopulation = ColorFactory.Instance.ConstructColorRamp(
        ColorRampAlgorithm.LinearContinuous,
        ColorFactory.Instance.RedRGB, ColorFactory.Instance.BlueRGB);

  //This Arcade expression categorizes the POP2000 field in the usaStates layer
  //into Low, Medium, and High values
  var arcade = @"if ($feature.POP2000 < 1000000)  
                   {return ""Low"";}" +
               @"else if ($feature.POP2000 >= 100000 && $feature.POP2000 < 5000000) 
                  {return ""Medium""; }" +
                @"else if ($feature.POP2000 >= 5000000) 
                  {return ""High"";}" +
                @"else{return ""NA"";}";
  //Create the Arcade expression info
  CIMExpressionInfo arcadeExp = new CIMExpressionInfo();
  arcadeExp.Expression = arcade;
  arcadeExp.ReturnType = ExpressionReturnType.Default;
  arcadeExp.Title = "State Population Categorized";
  arcadeExp.Name = "USA States Population 2000";

  //Each population category is assigned a color from the color ramp and symbolized on the map
  // using a unique value renderer
  var uvr_desc = new UniqueValueRendererDefinition()
  {
    //ValueFields = new List<string> { "STATION" },
    ColorRamp = colorRampForPopulation,
    UseDefaultSymbol = true,
    ValuesLimit = 100,
    ArcadeExpression = arcadeExp.Expression
  };
  //Create the renderer and assign it to the layer
  var renderer = usaStatesLayer.CreateRenderer(uvr_desc);
  usaStatesLayer.SetRenderer(renderer);