You can book your National Express tickets on emirates.com, an online travel agency or any travel agent.
When you book your journey, simply select the respective city as departure or destination as you would if you were booking a normal flight. You’ll find a combination of trains and flights that match your search criteria. National Express buses will either have an EK flight number between 3090 and 3121 or 9B flight number between 0001 and 0999.