You will also see the "Add new" button. Clicking on the button will open 2 options form: Amount input box for the amount of pre-defined tip and Value/Percentage choice for selection of the type of the service charge/tip. A common practice is to set 15% tip which will result in showing a quick selection button for a 15% service charge at the front end booking form third (final) step of the booking process.
Create as many pre-defined service charge buttons as you wish.