question

If you look at PlayFab's docs and APIs, you will see we are not using custom Id to integrate authentication with Steam or Google. For Steam, we will need a "SteamTicket" and for Google, we will need the "ServerAuthCode". They are not providing a "custom id" for linking on PlayFab.

Once your auth system can return unique Id for each player, you can then use custom Id as a bridge to connect your auth system with PlayFab. But it's highly not recommended. Custom Id is easy to fake, and hackers may pass your auth system to directly use the Id to log into PlayFab. It's not possible to validate that ID, nor is it possible to prevent a hacked client from putting whatever it wants in that ID.

Thus, to become a custom authentication provider like Steam or Google, we'd suggest your auth system implement OpenID Connect protocol. If your auth system can act as OpenID Provider, new game makers can use CreateOpenIdConnection to connect a PlayFab title with your company's platform (this step is like setting up Steam or Google add-ons) and then use LoginWithOpenIdConnect/LinkOpenIdConnect in client to link your platform's user account on PlayFab.

If it's hard for you to implement OpenID Connect protocol in your auth system, you can consider using LoginWithServerCustomId in your server and return SessionTicket/EntityToken to client directly.