Create a button when the page loads.

   
  

<StackPanel
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  x:Class="WpfApplication1.FELoaded" Loaded="OnLoad" Name="root">
</StackPanel>
//File:Window.xaml.cs

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

namespace WpfApplication1 {
    public partial class FELoaded {
        void OnLoad(object sender, RoutedEventArgs e)
        {
            Button b1 = new Button();
            b1.Content = "New Button";
            root.Children.Add(b1);
            b1.Height = 25;
            b1.Width = 200;
            b1.HorizontalAlignment = HorizontalAlignment.Left;
        }
    }
}