question

Sorry for the inconvenience, the LinkSteamId API is not included into Leagcy CouldScript, we recommend calling the API in AzureFunction. You may refer to PlayFab CloudScript using Azure Functions - PlayFab | Microsoft Learn to have more information about how to use AzureFunction. Otherwise, you have to send http request manually. And here is the working Leagcy CouldScript you may refer to:

 handlers.HttpTest=function(args,context)
 {
 var url = https://YourTitleId.playfabapi.com/Server/LinkSteamId; // Define the API endpoint URL
            
 // Define the request headers
 var headers = {
     "X-SecretKey": "Your Developer Secret Key" //This is what we pulled from args in previously
 };
            
 //Define data
 var data = {
     "PlayFabId": currentPlayerId,
     "SteamId": args.SteamId,// you need to pass in the SteamId
 };
    
 //Log stuff
 const json_data = JSON.stringify(data);
 var contentType="application/json"
    
 log.debug(`Data: ${json_data}`);
    
     // Make the request
 var reponse=http.request(url, "POST", json_data,contentType,headers);
    
 return {reponse}
    
 }