Exactly how you do this depends on the exact environment you are working in: for a website, I would probably do it in JavaScript as it would be a lot more responsive than doing the check at the server.
But...you can do it in C# (and it will work in web or windows) by handling the Click event for the button, and checking the Text property of the textbox:
if (myTextBox.Text == "This is Codeproject")
{
...
}
else
{
...
}
We can't tell you how to display it: we don't know enough about your system.