This question belong to what one man already asked Inviting / joining steam friends - Platform & Builds - Epic Developer Community Forums
Is here any way to get access to FOnlineAsyncTaskManagerSteam in custom C++ class that hold all methods, steam user ID and so on?
As I explained in my answer, you don’t need to access FOnlineAsyncTaskManagerSteam unless you want some very custom interfacing with Steam.
IOnlineSubsystem is probably what you are looking for to access all the online methods (Friends, Server Listings, Achievements), steam user ID and so on.