public class CIMChartPieSeries : CIMChartSeries, System.ComponentModel.INotifyPropertyChanged, System.Xml.Serialization.IXmlSerializable  

// For more information on the chart CIM specification:
// https://github.com/Esri/cim-spec/blob/main/docs/v3/CIMCharts.md

// Define fields names used in chart parameters.
const string fieldCategory = "neighbourhood_group";

var lyrs = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>();
var lyr = lyrs.First();
var lyrDef = lyr.GetDefinition();

// Define pie chart CIM properties
var pieChart = new CIMChart
{
   Name = "pieChart",
   GeneralProperties = new CIMChartGeneralProperties
   {
      Title = "Pie chart with custom legend formatting",
      UseAutomaticTitle = true
   },
   Legend = new CIMChartLegend
   {
      LegendText = new CIMChartTextProperties
      {
         FontFillColor = new CIMRGBColor { R = 0, G = 250, B = 20 }, // Specify new font color
         FontSize = 6.0, // Specify new font size
      }
   },
   Series = new CIMChartSeries[] {
      new CIMChartPieSeries
      {
         UniqueName = "pieSeries",
         Name = "pieSeries",
         Fields = new string[] { fieldCategory, string.Empty }
      }
   }
};

// Add new chart to layer's existing list of charts (if any exist)
var newCharts = new CIMChart[] { pieChart };
var allCharts = (lyrDef?.Charts == null) ? newCharts : lyrDef.Charts.Concat(newCharts);
// Add CIM chart to layer defintion 
lyrDef.Charts = allCharts.ToArray(); 
lyr.SetDefinition(lyrDef);