Class EingabeForm
 
  Private Sub EingabeForm_FormClosing(sender As Object, e As _
    FormClosingEventArgs) Handles Me.FormClosing
    e.Cancel = e.CloseReason = CloseReason.UserClosing
  End Sub
 
  Private Sub Button1_Click(sender As Object, e As EventArgs) Handles _
    Button1.Click
    If Me.TextBox1.Text = "frei" Then
      Application.Exit()
    End If
  End Sub
 
End Class

  Private Sub SperrForm_Shown(sender As Object, e As EventArgs) Handles Me.Shown
    Me.Timer1.Start()
    EingabeForm.ShowDialog(Me)
  End Sub
 
  Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
    'Reihenfolge nicht ändern!
    Me.BringToFront()
    EingabeForm.BringToFront() 'Wichtig!
  End Sub