question

I assume that you’re referring to RequestMultiplayerServer, is that correct? In that case, the parameter “SessionCookie” can be used to send some data to the requested server. And, in the requested server, use the GSDK to retrieve the data from the session cookie:

if (config.TryGetValue(GameserverSDK.SessionCookieKey,
out string sessionCookie))
{
// sessionCookie
contains the value
}

Integrating game servers with the PlayFab Game Server SDK (GSDK) - PlayFab | Microsoft Docs