Add-Type -AssemblyName System.Windows.Forms

function button1_click() {
[Windows.Forms.MessageBox]::Show("hello!")
}

$form1 = New-Object System.Windows.Forms.Form
$form1.Text = "greeting"
$button1 = New-Object System.Windows.Forms.Button
$button1.Text = "hello"
$button1.Top = 100
$button1.Left = 100
$button1.Add_Click({button1_click})
$form1.Controls.Add($button1)
$form1.ShowDialog()