An About Form

I am trying to create an about form in my application. How to you make it so it behaves like the about form in notepad, for example. Here is how I describe the behavior of the about form in notepad.

When the about form in notepad is open, the main application is grayed out and cannot receive focus until the about form is closed. Therefore when you click notepad on the task bar, the about form jumps into focus, not notepad.  How do you code you C# application and about form to behave like this?