Let me know!
Right now the only way is for 343 to gift it to you and they are currently gifting it to a select forgers. From last week’s Community Update:
> Specifically, everyone who creates a map or mode that makes its way to Matchmaking will be granted the “Olive” helmet as a badge of honor and token of thanks. We will start manually granting these to Forgers with existing content in Matchmaking soon and then going forward, will issue the “Olive” helmet out as we ingest new Forge content into Halo 5’s playlists.
Locking this down as there is a large thread in the Halo 5 forums discussing it just now