WPF/MVVM drag-and-drop process selector.

ProcessSelector.xaml

<UserControl x:Class="Ace.UI.Windows.Controls.ProcessSelector"
             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" 
             xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes"
             xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
             xmlns:viewModels="clr-namespace:Ace.UI.Windows.ViewModels"
             mc:Ignorable="d">
    <UserControl.DataContext>
        <viewModels:ProcessSelectorViewModel />
    </UserControl.DataContext>

    <StackPanel>
        <Grid Margin="0 0 0 8">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition />
            </Grid.ColumnDefinitions>
            <Grid x:Name="Target" Grid.Column="0" Background="Transparent" MouseDown="Target_MouseDown" MouseUp="Target_MouseUp">
                <i:Interaction.Triggers>
                    <i:EventTrigger EventName="MouseUp">
                        <i:InvokeCommandAction Command="{Binding SelectCommand}" />
                    </i:EventTrigger>
                </i:Interaction.Triggers>
                <materialDesign:PackIcon Kind="Target" Width="36" Height="36" Foreground="{DynamicResource PrimaryHueDarkBrush}" />
            </Grid>
            <Grid Grid.Column="1" Margin="8 0 0 0" VerticalAlignment="Stretch">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>
                <Grid.RowDefinitions>
                    <RowDefinition />
                    <RowDefinition />
                </Grid.RowDefinitions>
                <TextBlock Grid.Column="0" Grid.Row="0" VerticalAlignment="Center" Text="Process ID:" />
                <TextBlock Grid.Column="1" Grid.Row="0" VerticalAlignment="Center" Text="{Binding Path=SelectedProcess.Id}" />
                <TextBlock Grid.Column="0" Grid.Row="1" VerticalAlignment="Center" Text="Process Name:" />
                <TextBlock Grid.Column="1" Grid.Row="1" VerticalAlignment="Center" Text="{Binding Path=SelectedProcess.ProcessName, StringFormat={}{0}.exe}" />
            </Grid>
        </Grid>
        <TextBlock HorizontalAlignment="Center" Text="Drag the target icon over the game window." />
    </StackPanel>
</UserControl>

ProcessSelector.xaml.cs

using System.Windows.Controls;
using System.Windows.Input;

namespace Ace.UI.Windows.Controls {
    /// <summary>
    /// Interaction logic for ProcessSelector.xaml
    /// </summary>
    public partial class ProcessSelector : UserControl {
        public ProcessSelector() {
            InitializeComponent();
        }

        private void Target_MouseDown(object sender, MouseButtonEventArgs e) {
            Target.CaptureMouse();
            Mouse.OverrideCursor = Cursors.Hand;
        }

        private void Target_MouseUp(object sender, MouseButtonEventArgs e) {
            Target.ReleaseMouseCapture();
            Mouse.OverrideCursor = null;
        }
    }
}

ProcessSelectorViewModel.cs

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Drawing;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Windows.Input;
using Prism.Commands;

namespace Ace.UI.Windows.ViewModels {
    public class ProcessSelectorViewModel : INotifyPropertyChanged {
        #region Imports
        [DllImport("user32.dll")]
        private static extern IntPtr WindowFromPoint(Point p);

        [DllImport("user32.dll")]
        private static extern IntPtr GetAncestor(IntPtr hwnd, uint flags);

        [DllImport("user32.dll")]
        private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out int processId);
        #endregion

        #region Fields
        private Process _selectedProcess;
        #endregion

        #region Properties
        public ICommand SelectCommand { get; }

        public Process SelectedProcess {
            get { return _selectedProcess; }
            private set { _selectedProcess = value; NotifyPropertyChanged(); }
        }
        #endregion

        public ProcessSelectorViewModel() {
            SelectCommand = new DelegateCommand(UpdateSelectedProcess);
        }
        
        private void UpdateSelectedProcess() {
            var window = GetAncestor(WindowFromPoint(Control.MousePosition), 2);
            
            GetWindowThreadProcessId(window, out var processId);
            var process = Process.GetProcessById(processId);

            if (process.Id != Process.GetCurrentProcess().Id)
                SelectedProcess = process;
        }

        #region INotifyPropertyChanged Implementation
        public event PropertyChangedEventHandler PropertyChanged;

        private void NotifyPropertyChanged([CallerMemberName] string propertyName = null) {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion
    }
}