question

I came up with an short term solution:

1. Add this to your cloud script:

handlers._twitchGenericId = function(args, context) {
    var request = {
        PlayFabId: currentPlayerId
    };
    var result = server.GetUserAccountInfo(request);
    if (result["UserInfo"] && result["UserInfo"]["TwitchInfo"])
    {
        server.AddGenericID({
           PlayFabId: currentPlayerId,
           GenericId: {
               ServiceName: "Twitch",
               UserId: result["UserInfo"]["TwitchInfo"]["TwitchId"]
           }
        });
    }
}

2. Add a new segment with the following arguments:

3. Add an "Entered Segment" action of type "Execute Cloud Script" and select your cloud script.

4. Then whenever you're asking for the user on the server, use https://[yourTitleId].playfabapi.com/Server/GetPlayFabIDsFromGenericIDs

Make sure your body is setup like this:

{
    "GenericIDs": [
        {
            "ServiceName": "Twitch",
            "UserId": 204940653
        }
    ]
}

--------------------------------------

Now, whenever you get a response for the generic ID, you'll get back the user's playfab ID.

While this is nice, it's not an official solution, I'd much prefer there be a way to grab the playfab ID from twitch user ID on the server; just like you can with all the other services.