You can use the PSScriptPad WPF Designer from the PowerShell Pro Tools Visual Studio Code extension. It can be opened using the Show Form Designer button found on XAML files.

Create a new WPF Form

To create a new WPF Form, first create a .xaml file in your workspace. Next, put the default Window code found below and save the XAML file.

<Window 
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"
mc:Ignorable="d"
x:Name="wpfWindow" Background="White" Title="WPF Window">
​
</Window>

Next, click the Show Form Designer button in the toolbar of the file. PSScriptPad will open the XAML file in the WPF Designer.