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()
  {
    //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();
    //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()
  {
    //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
  }
}

{
  // 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();
    //If TextSymbol is a call-out the leader line anchor point can be modified
    theLabelClass.MaplexLabelPlacementProperties.PolygonAnchorPointType = MaplexAnchorPointType.Perimeter;
    lyrDefn.LabelClasses = [.. listLabelClasses]; //Set the labelClasses back
    featureLayer.SetDefinition(lyrDefn); //set the layer's definition
  }
}