In Visual Studio:

File -> New -> Project
Installed -> Templates -> Visual C# -> Windows -> WPF Application

Ersetze den Code in MainWindow.xaml durch:

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:WpfApplication2"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525"
    >
    <StackPanel>
        <TextBlock Text="Enter some text">
        <TextBox x:Name="ResponseTextBox">
        <Button Content="OK" Click="OKButton_Click">
    </StackPanel>
</Window>

Ersetze den Code in MainWindow.xaml.cs durch:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    public string ResponseText
    {
        get { return ResponseTextBox.Text; }
        set { ResponseTextBox.Text = value; }
    }

    private void OKButton_Click(object sender, System.Windows.RoutedEventArgs e)
    {
        MessageBox.Show("You said: " + ResponseText);
    }
}

Abgeleitet aus: http://stackoverflow.com/a/2796487/1777526

Dann einfach starten und ausprobieren.
Weitere Controls kann man durch drag&drop aus der Toolbox in den XAML-Viewer hinzufügen.