public double MaximumCharacterSpacing {get; set;}

{
  // Note: call within QueuedTask.Run()
  {
    //Check if the label engine is Maplex or standard.
    CIMGeneralPlacementProperties labelEngine = MapView.Active.Map.GetDefinition().GeneralPlacementProperties;
    if (labelEngine is CIMStandardGeneralPlacementProperties)
      return;

    //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();
    //Spread Labels (words and characters to fill feature)
    // Spread words to fill feature
    theLabelClass.MaplexLabelPlacementProperties.SpreadWords = true;
    //Spread Characters to a fixed limit of 50%
    theLabelClass.MaplexLabelPlacementProperties.SpreadCharacters = true;
    theLabelClass.MaplexLabelPlacementProperties.MaximumCharacterSpacing = 50.0;
    lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
    featureLayer.SetDefinition(lyrDefn); //set the layer's definition
  }
}