protected EmbeddableControl OverlayEmbeddableControl {get;}

internal class MapTool_WithOverlayControl : MapTool
{
  public MapTool_WithOverlayControl()
  {
    OverlayControlID = "mapTool_EmbeddableControl";
  }

  protected override void OnToolMouseDown(MapViewMouseButtonEventArgs e)
  {
    e.Handled = true;
  }

  protected override Task HandleMouseDownAsync(MapViewMouseButtonEventArgs e)
  {
    //Get the instance of the ViewModel
    var vm = OverlayEmbeddableControl as EmbeddedControlViewModel;
    if (vm == null)
      return Task.FromResult(0);

    //Get the map coordinates from the click point and set the property on the ViewMode.
    return QueuedTask.Run(() =>
    {
      var mapPoint = MapView.Active.ClientToMap(e.ClientPoint);
      vm.ClickText = string.Format("X: {0}, Y: {1}, Z: {2}", mapPoint.X, mapPoint.Y, mapPoint.Z);
    });
  }
}

/*

   Copyright 2018 Esri

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

   See the License for the specific language governing permissions and
   limitations under the License.

*/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using ArcGIS.Desktop.Framework;
using ArcGIS.Desktop.Framework.Contracts;
using ArcGIS.Desktop.Framework.Controls;
using System.Xml.Linq;

namespace Examples
{
  /// <summary>
  /// Derive from ArcGIS.Desktop.Framework.Controls.EmbeddableControl
  /// </summary>
  internal class EmbeddedControlViewModel : EmbeddableControl
  {
    public EmbeddedControlViewModel(XElement options, bool canChangeOptions) : base(options, canChangeOptions)
    {
    }

    private string _clickText = "Click in view to show coordinates";
    public string ClickText
    {
      get { return _clickText; }
      set
      {
        SetProperty(ref _clickText, value, () => ClickText);
      }
    }
  }
}