Press and hold the speed dial button that you want to configure. Enter the name and extension (or external number, don't forget the 9 and area code!) and press Save.
Note: If you also want the button to show when the person is on the phone, you will first need to set the speed dial button, and then contact IT to configure that setting. Just tell us what you called the button, and that you want to "enable BLF" (Busy Lamp Field).