What i did - i’ve crated NetId mannually.
FUniqueNetIdPtr unid = (FUniqueNetIdRef)FUniqueNetIdString::Create(FString::Printf(TEXT("Dedicated-Server%d"), 0), NULL_SUBSYSTEM);
But then it turns out that FOnlineSessionNull::CreateSession desnt even need it:
bool FOnlineSessionNull::CreateSession(const FUniqueNetId& HostingPlayerId, FName SessionName, const FOnlineSessionSettings& NewSessionSettings)
{
// todo: use proper HostingPlayerId
return CreateSession(0, SessionName, NewSessionSettings);
}