The route, session and ALL dependencies have to be built-in or on the DLS. Even one blade of grass on a third-party website will mean that the route and session cannot be used for Multipayer at all!
Once you have the route built and done, you can upload it to the DLS. Once there, others can download it and enjoy it. Then once you have a session uploaded that has the multiplayer rule in it, others can join you, or you can join others games.