[messry]

كود :

   Private Sub RichTextBox1_TextChanged(sender As Object, e As EventArgs) Handles RichTextBox1.TextChanged
       If RichTextBox1.Text = "Activation was successful" Then
           MsgBox("Teeeeeeeeeeeeeeeeet")
       End If
   End Sub

ارفق المثال وان شاء الله اعدل عليه