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

  var lyrDef = featureLayer.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 CIMChartPieSeries
            {
               UniqueName = "pieSeries",
               Name = "pieSeries",
               Fields = new string[] { categoryField, 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 definition 
  lyrDef.Charts = allCharts.ToArray();
  featureLayer.SetDefinition(lyrDef);
}