Hello @vaughanmatt,
Thanks for writing to us.
In case if you are using Pro Header builder in that case you can create two types of menu one for desktop and another one for Mobile menu. After creating two menus you can display the menu through Navigation Inline for desktop and another menu you can display through Navigation dropdown element for a mobile device where you can add the “Book Online” link as your menu item.
After setting this Navigation Inline element and Navigation Dropdown element in the header you have to hide it by using Breakpoints. In case if you have no idea about how to hide content through breakpoint please have a look at our doc.
Hope it helps
Thanks