just open this link A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums read it’s content and do it’s steps to enable steam online subsystem in UE4
it would be better to download steam sdk Steamworks because enabling steam online subsystem require matching steam version in engine folder name
if UE4 4.8.0 compiled with steam version 132, then you should use steam dlls of same version, of course you can try even higher version dlls, maybe it have 100% backward compatibility