Unlike other titles in the franchise, Halo 5 was never ported to Steam or the Windows Store. Plans for a port were reportedly scrapped by 343 Industries due to "significant technical hurdles" related to the engine.
If you want to experience the story of Spartan Locke and Master Chief on PC, your only official options are: Xbox Cloud Gaming: Halo 5 Pc Game -repack By R G Mechanics- Torrents