Don't you think it would be a good idea for you to understand what is going on and how to do things
before you start writing tutorials?
I'm pretty sure that any tutorial written by a beginner is going to miss out a lot of important stuff. For example, the lack of use of the StringBuilder class in your example, the use of Visual studio default names...
Try this:
textBox2.Text = "Hello" + Environment.NewLine & "Goodbye"