The Marshadow event is now unavailable, it was only an event for Sun/Moon last October. There is currently no way to obtain Marshadow in Ultra Sun/Ultra Moon. The only way to get Marshadow without the event is by hacking or by using the GTS.
You don't. Marshadow was only available as an event from Pokémon Sun/Moon. I doubt Game Freak will make a second chance event for Marshadow. The only other ways to get Marshadow would be through a Link Trade or the GTS.
I hope I helped.