public string UpperLabel {get; set;}

{
    string colorBrewerSchemesName = "ArcGIS Colors";
    //Get the style project item that contains the color ramps
    //Refer to the Initialize region for an example of how to get a style item
    //and the color ramp from it.
    //Note: Run within a QueuedTask
    //defining a heatmap renderer that uses values from Population field as the weights
    HeatMapRendererDefinition heatMapDef = new HeatMapRendererDefinition()
    {
        Radius = 20,
        WeightField = "Population",
        ColorRamp = colorRamp,
        RendereringQuality = 8,
        UpperLabel = "High Density",
        LowerLabel = "Low Density"
    };
    CIMHeatMapRenderer heatMapRndr = featureLayer.CreateRenderer(heatMapDef) as CIMHeatMapRenderer;
    featureLayer.SetRenderer(heatMapRndr);
}