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

Hello,

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.