The Popup Message behavior displays a JavaScript alert with the message you specify. Because JavaScript alerts have only one button (OK), use this behavior to give the user information rather than to present a choice.
You can embed any valid JavaScript function call, property, global variable, or other expression in the text. To embed a JavaScript expression, place it inside braces ({}). To display a brace, precede it with a backslash (\{).
Example:
The URL for this page is {window.location}, and today is {new Date()}.