public void SetFilterGeometry( 
   Geometry filterGeometry
)

          // Note: Run within QueuedTask.Run
          // Set the spatial filter geometry

          //Get the geometry from the selected features in the feature layer
          var spatialClauseGeom = spatialDefnLayer.GetFeatureOutline(MapView.Active, FeatureOutlineType.Selected);

DefinitionQuery definitionQuery = new DefinitionQuery
{
  WhereClause = whereClause,
  Name = $"{layerToQuery.Name}"
};
//Setting the spatial filter to the Definition Query
if (definitionQuery.CanSetFilterGeometry(spatialClauseGeom))
{
  definitionQuery.SetFilterGeometry(spatialClauseGeom);
}

layerToQuery.InsertDefinitionQuery(definitionQuery);
layerToQuery.SetActiveDefinitionQuery(definitionQuery.Name);

    var greatLakes = map.GetLayersAsFlattenedList()
                .OfType<FeatureLayer>().First(l => l.Name == "Great Lakes");
    var usa_states = map.GetLayersAsFlattenedList()
    .OfType<FeatureLayer>().First(l => l.Name == "US_States");

    QueuedTask.Run(() =>
    {
//name must be unique
        var def_query = new DefinitionQuery("GreatLakes",
                                                    "NAME in ('Huron','Michigan','Erie')");

        //create a filter geometry - in this example we will use the outline geometry
        //of all visible features from a us states layer...the filter geometry will be
//intersected with the layer feature geometry when added to the def query
        var filter_geom = usa_states.GetFeatureOutline(mv, FeatureOutlineType.Visible);
        //other options...
        //var filter_geom = usa_states.GetFeatureOutline(mv, FeatureOutlineType.All);
        //var filter_geom = usa_states.GetFeatureOutline(mv, FeatureOutlineType.Selected);

        //Geometry must have a valid SR and be point, multi-point, line, or poly
        if (def_query.CanSetFilterGeometry(filter_geom))
{
  def_query.SetFilterGeometry(filter_geom);
}

        //Apply the def query
        greatLakes.InsertDefinitionQuery(def_query);
        //Set it active
        greatLakes.SetActiveDefinitionQuery(def_query.Name);

        //or....also - set it active when it is inserted
        //greatLakes.InsertDefinitionQuery(def_query, true);
    });