public CIMMaplexDictionaryEntry[] MaplexDictionary {get; set;}

public static void CreateDictionary()
{
  //Get the map's definition
  var mapDefn = MapView.Active.Map.GetDefinition();
  //Get the Map's Maplex labelling engine properties
  var mapDefnPlacementProps = mapDefn.GeneralPlacementProperties as CIMMaplexGeneralPlacementProperties;

  //Define the abbreviations we need in an array            
  List<CIMMaplexDictionaryEntry> abbreviationDictionary = new List<CIMMaplexDictionaryEntry>
        {
            new CIMMaplexDictionaryEntry {
            Abbreviation = "Hts",
            Text = "Heights",
            MaplexAbbreviationType = MaplexAbbreviationType.Ending

         },
            new CIMMaplexDictionaryEntry
            {
                Abbreviation = "Ct",
                Text = "Text",
                MaplexAbbreviationType = MaplexAbbreviationType.Ending

            }
            //etc
        };
  //The Maplex Dictionary - can hold multiple Abbreviation collections
  var maplexDictionary = new List<CIMMaplexDictionary>
        {
            new CIMMaplexDictionary {
                Name = "NameEndingsAbbreviations",
                MaplexDictionary = abbreviationDictionary.ToArray()
            }

        };
  //Set the Maplex Label Engine Dictionary property to the Maplex Dictionary collection created above.
  mapDefnPlacementProps.Dictionaries = maplexDictionary.ToArray();
  //Set the Map definition 
  MapView.Active.Map.SetDefinition(mapDefn);
}

private static void ApplyDictionary()
{
  var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().First();

  QueuedTask.Run(() =>
  {
    //Creates Abbreviation dictionary and adds to Map Definition                                
    CreateDictionary();
    //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 props to use abbreviation dictionary 
    CIMGeneralPlacementProperties labelEngine = MapView.Active.Map.GetDefinition().GeneralPlacementProperties;
    theLabelClass.MaplexLabelPlacementProperties.DictionaryName = "NameEndingsAbbreviations";
    theLabelClass.MaplexLabelPlacementProperties.CanAbbreviateLabel = true;
    theLabelClass.MaplexLabelPlacementProperties.CanStackLabel = false;
    //Set the labelClasses back
    lyrDefn.LabelClasses = listLabelClasses.ToArray();
    //set the layer's definition
    featureLayer.SetDefinition(lyrDefn);
  });
}