public Geometry GetFeatureOutline( 
   MapView view,
   FeatureOutlineType outlineType
)

{
    var layers = map.GetLayersAsFlattenedList().OfType<FeatureLayer>();
    var layerToQuery = layers.FirstOrDefault(f => f.Name == "USNationalParks");
    if (layerToQuery == null) return;

    string whereClause = "RecreationVisitsTotal > 1000000"; //More than million visitors a year

    var spatialDefnLayer = layers.FirstOrDefault(f => f.Name == "AllUSStates");
    if (spatialDefnLayer == null) return;
    try
    {
        if (MapView.Active == null) return;
        //Note: needs to be called on the MCT
        // Set the spatial filter geometry
        //Get the geometry from the selected features in the feature layer
        var spatialClauseGeom = spatialDefnLayer.GetFeatureOutline(MapView.Active, FeatureOutlineType.Selected);
        //Create the definition query
        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);
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine($"Error querying feature layer: {ex.Message}");
    }
}

{
    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");
    //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(mapView, FeatureOutlineType.Visible);
    //other options...
    //var filter_geom = usa_states.GetFeatureOutline(mapView, FeatureOutlineType.All);
    //var filter_geom = usa_states.GetFeatureOutline(mapView, FeatureOutlineType.Selected);

    //Geometry must have a valid SR and be point, multi-point, line, or poly
    //Note: needs to be called on the MCT
    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);
}

{
    var greatLakes = map.GetLayersAsFlattenedList()
          .OfType<FeatureLayer>().First(l => l.Name == "Great Lakes");
    var michigan = map.GetBookmarks().First(b => b.Name == "Michigan");

    //get all features - multiple feature geometries are always returned as a
    //single multi-part
    var all_features_outline = greatLakes.GetFeatureOutline(mapView, FeatureOutlineType.All);

    //or get just the outline of selected features
    var qry = new QueryFilter()
    {
        SubFields = "*",
        WhereClause = "NAME in ('Huron','Michigan','Erie')"
    };
    greatLakes.Select(qry);
    var sel_features_outline = greatLakes.GetFeatureOutline(
        mapView, FeatureOutlineType.Selected);
    greatLakes.ClearSelection();

    //or just the visible features
    mapView?.ZoomTo(michigan);
    var visible_features_outline = greatLakes.GetFeatureOutline(
        mapView, FeatureOutlineType.Visible);
}