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.
Source: Knowledge
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.
