I just tested this, and it works fine.
private void Test_Click(object sender, RoutedEventArgs e)
{
var window = new Window();
var stackPanel = new StackPanel { Orientation = Orientation.Vertical };
stackPanel.Children.Add(new TextBox { Text = "Text" });
window.Content = stackPanel;
window.Show();
}
<Window x:Class="WPF_Test.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Button x:Name="Test" Content="Click Me" Click="Test_Click" />
</Grid>
</Window>
When you click on the button in the Main Window, it opens up a new window with a text box which can be edited.