Public Class Form1

    Public RetString As String

    Private Sub Button1_Click(ByVal sender As System.Object, _
                              ByVal e As System.EventArgs) Handles Button1.Click

        Me.DialogResult = System.Windows.Forms.DialogResult.OK
        RetString = Me.TextBox1.Text
        Me.Close()

    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, _
                           ByVal e As System.EventArgs) Handles MyBase.Load

        Me.Activate()

    End Sub

End Class

Add-Type -path "Form1.vb","Form1.Designer.vb" -ReferencedAssemblies System.Windows.Forms,System.Drawing
$obj = New-Object Form1
$ret = $obj.ShowDialog()

$ret
$obj.RetString
$obj.Dispose()