Playfab Online Subsystem and Steam Online Subsystem (any help would be highly appreciated)

Hello Everyone ,
So i have been stuck in this for way too much and i need some help
so we want to create our own lobbies and in-game chat system
we are using playfab dedicated servers so we gonna need to use playfab online subsystem
but currently we are using steam online subsystem for login and payments
and i’m stuck here , if i set playfab as default online subsystem , i fail to get steam auth ticket to login using steam
i found this article
link
to use 2 online subsystems but i failed to apply it
any help would be highly appreciated

Grab the Steamworks SDK and make calls to it directly.

thanks for your reply but can you further explain this please