How to send custom FUniqteNetId

AGameModeBase::PreLogin has a FUniqueNetIdRepl parameter which is a random generated number + my desktop name. (because I am not using any online subsystem)
I want to send my own FUniqueNetIdRepl from client. I there any easy way like overriding a function or should I write my own online subsystem which is a pain in UE4. :frowning: