Sending variable from server to the Game Instance after finishing an event

I am currently testing ways to make a auth system for the online game that I will try to make. Currently, I have VaRest plugin installed and the php file on my server with the GET request. My concern is that if I run GET request from the client, user can intercept or even change that request via software like fiddler or similar.
Now, I want to know if there is a way to send a GET request on the server, with variables taken from my Login menu, check my database for user credentials and then save bool as true or false, but on GAME INSTANCE.
For now, I only managed to get that variable saved if I play as Listener server and the variable is not printed on client.
Thank you.

For a start you should put things like password and username in the POST body and don’t use GET. Also use HTTPS. There’s a lot of information about the security of that online, as it’s not just an Unreal issue obviously. And there’s no reason that wouldn’t work through Game Instance, can you post your Blueprint, I’m not sure how you’re currently doing it.

Hello, thank you for replying !

Yes, I did tried to use POST requests, but to be honest, I have NO IDEA how to make one using VaREST plugin or how to make a php file for it and the documentation for it is not really helpful.
Please note that I am just testing it and here are the blueprints

Also, I tried using Multicast event, but whatever I do, it seems that the Server side can make the variable ( I guess due to authority switch ) but the client cannot.
I know that I am doing this wrong, but any help would be appreciated . Simple, event for printing is added on the instance for testing.

EDIT : I used SSL server side, but still, Fiddler can decrypt any data, even HTTPS over the network that is sent from client, even POST body, which is why I want this to be done on the server side and then true of false transfered to the client.