public CIMMaplexLabelPlacementProperties MaplexLabelPlacementProperties {get; set;}

//Note: call within QueuedTask.Run()

//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();

//Modify label Placement 
//Check if the label engine is Maplex or standard.
CIMGeneralPlacementProperties labelEngine = 
   MapView.Active.Map.GetDefinition().GeneralPlacementProperties;
if (labelEngine is CIMStandardGeneralPlacementProperties) //Current labeling engine is Standard labeling engine               
  theLabelClass.StandardLabelPlacementProperties.PointPlacementMethod = 
           StandardPointPlacementMethod.OnTopPoint;
else    //Current labeling engine is Maplex labeling engine            
  theLabelClass.MaplexLabelPlacementProperties.PointPlacementMethod = 
          MaplexPointPlacementMethod.CenteredOnPoint;

lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition

//Note: call within QueuedTask.Run()

//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();
//Modify label Rotation
CIMMaplexRotationProperties rotationProperties = new CIMMaplexRotationProperties
{
  Enable = true, //Enable rotation
  RotationField = "ELEVATION", //Field that is used to define rotation angle
  AdditionalAngle = 15, //Additional rotation 
  RotationType = MaplexLabelRotationType.Arithmetic,
  AlignmentType = MaplexRotationAlignmentType.Perpendicular,
  AlignLabelToAngle = true
};
theLabelClass.MaplexLabelPlacementProperties.RotationProperties = rotationProperties;
lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition

//Note: call within QueuedTask.Run()

//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

//Note: call within QueuedTask.Run()

//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();
//If TextSymbol is a callout the leader line anachor point can be modified
theLabelClass.MaplexLabelPlacementProperties.PolygonAnchorPointType = MaplexAnchorPointType.Perimeter;
lyrDefn.LabelClasses = listLabelClasses.ToArray(); //Set the labelClasses back
featureLayer.SetDefinition(lyrDefn); //set the layer's definition