ArcGIS Pro 2.7 API Reference Guide
OverlayControlID Property
Example 

ArcGIS.Desktop.Mapping Namespace > MapTool Class : OverlayControlID Property
Gets or sets the DAML ID of the embeddable control to show on the map view when the tool is active.
Syntax
protected string OverlayControlID {get; set;}
Protected Property OverlayControlID As String
Remarks
In some cases you may need to provide a control that displays on top of the map view to configure the behavior of the tool while in use similar to the measure tool. This property allows you to define a Embeddable Control to display on top of the map view while to the tool is active. When you activate a different tool the overlay control will be removed. You can get a reference to the Embeddable Control using the OverlayEmbeddableControl property.
Example
Set the overlay embeddable control for a MapTool.
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.

-->
<UserControl x:Class="Examples.EmbeddedControlView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"                                             
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <TextBlock Text="{Binding ClickText}" HorizontalAlignment="Center"/>
    </Grid>
</UserControl>
/*

   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);
      }
    }
  }
}
<?xml version="1.0" encoding="utf-8" ?>
<!--

   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.

-->
<ArcGIS defaultAssembly="Examples.dll" defaultNamespace="Examples" xmlns="http://schemas.esri.com/DADF/Registry" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://schemas.esri.com/DADF/Registry file:///C:/Program%20Files/ArcGIS/Pro/bin/ArcGIS.Desktop.Framework.xsd">
  <AddInInfo id="{40c18dbd-a8ac-46a1-93ae-4c3ccc618616}">
    <Name>Examples</Name>
    <Description>Examples</Description>
  </AddInInfo>

  <!--Define the embeddable control in the Config.daml-->
  <categories>
    <updateCategory refID="esri_embeddableControls">
      <insertComponent id="mapTool_EmbeddableControl" className="EmbeddedControlViewModel">
        <content className="EmbeddedControlView" />
      </insertComponent>
    </updateCategory>
  </categories>

</ArcGIS>
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

MapTool Class
MapTool Members