C++ API

Add the module to your .Build.cs:

PrivateDependencyModuleNames.AddRange(new string[] { "SteamCoreWeb" });

Calling a function

Get the subsystem from the Game Instance and call the function. The callback is a dynamic delegate, so bind it to a UFUNCTION.

#include "SteamUser/WebSteamUser.h"
#include "SteamCoreWeb/SteamWebUtilities.h"

void UMyFriendsComponent::RequestPlayerSummaries(const TArray<FString>& SteamIds)
{
	UWebSteamUser* SteamUser = GetWorld()->GetGameInstance()->GetSubsystem<UWebSteamUser>();
	if (!SteamUser)
	{
		return;
	}

	FOnSteamCoreWebCallback Callback;
	Callback.BindUFunction(this, GET_FUNCTION_NAME_CHECKED(UMyFriendsComponent, OnPlayerSummaries));

	SteamUser->GetPlayerSummaries(Callback, USteamWebUtilities::GetProjectKey(), SteamIds);
}

void UMyFriendsComponent::OnPlayerSummaries(const FString& Data, bool bWasSuccessful)
{
	if (!bWasSuccessful)
	{
		return;
	}

	TArray<FString> Names;
	ESteamJsonResult Result;
	USteamWebUtilities::FindJsonStrings(Data, TEXT("personaname"), Names, Result);
}

Declare the callback as a UFUNCTION:

UFUNCTION()
void OnPlayerSummaries(const FString& Data, bool bWasSuccessful);

The callback is called on the game thread. If the object is destroyed before Steam responds, the callback is not called.

Parsing the response with Unreal's JSON module

For more control than the JSON helpers give, parse Data with Unreal's JSON module. Add "Json" to your .Build.cs as well.

#include "Dom/JsonObject.h"
#include "Serialization/JsonReader.h"
#include "Serialization/JsonSerializer.h"

TSharedPtr<FJsonObject> Root;
const TSharedRef<TJsonReader<>> Reader = TJsonReaderFactory<>::Create(Data);

if (FJsonSerializer::Deserialize(Reader, Root) && Root.IsValid())
{
	const TSharedPtr<FJsonObject>* Response;
	if (Root->TryGetObjectField(TEXT("response"), Response))
	{
		const TArray<TSharedPtr<FJsonValue>>* Players;
		if ((*Response)->TryGetArrayField(TEXT("players"), Players))
		{
			for (const TSharedPtr<FJsonValue>& Player : *Players)
			{
				const FString Name = Player->AsObject()->GetStringField(TEXT("personaname"));
			}
		}
	}
}

Subsystem classes

Class	Header
UWebApps	Apps/WebApps.h
UWebBroadcastService	BroadcastService/WebBroadcastService.h
UWebCheatReporting	CheatReporting/WebCheatReporting.h
UWebEconMarketService	EconMarket/WebEconMarketService.h
UWebEconService	EconService/WebEconService.h
UWebGameInventory	GameInventory/WebGameInventory.h
UWebGameNotificationsService	GameNotificationsService/WebGameNotificationsService.h
UWebGameServersService	GameServersService/WebGameServersService.h
UWebInventoryService	InventoryService/WebInventoryService.h
UWebLeaderboards	Leaderboards/WebLeaderboards.h
UWebLobbyMatchmakingService	LobbyMatchmakingService/WebLobbyMatchmakingService.h
UWebMicroTxn	MicroTxn/WebMicroTxn.h
UWebPlayerService	PlayerService/WebPlayerService.h
UWebPublishedFileService	PublishedFileService/WebPublishedFileService.h
UWebSteamPublishedItemSearch	PublishedItemSearch/WebPublishedItemSearch.h
UWebPublishedItemVoting	PublishedItemVoting/WebPublishedItemVoting.h
UWebSteamCommunity	SteamCommunity/WebSteamCommunity.h
UWebSteamEconomy	SteamEconomy/WebSteamEconomy.h
UWebSteamGameServerStats	SteamGameServerStats/WebSteamGameServerStats.h
UWebSteamNews	SteamNews/WebSteamNews.h
UWebRemoteStorage	SteamRemoteStorage/WebRemoteStorage.h
UWebSteamUser	SteamUser/WebSteamUser.h
UWebSteamWebAPIUtil	WebAPIUtil/WebSteamWebAPIUtil.h
UWebStoreService	StoreService/WebStoreService.h
UWebUserAuth	SteamUserAuth/WebUserAuth.h
UWebUserStats	SteamUserStats/WebUserStats.h
UWebWishlistService	WishlistService/WebWishlistService.h
UWebWorkshop	Workshop/WebWorkshop.h