You can force your users to authenticate before they use your Linux proxy server using Apache authentication.
sudo touch /etc/squid/passwd
Change the ownership to squid daemon so it can access the file
sudo chown squid /etc/squid/passwd
Create a new user using the htpasswd command:
sudo htpasswd /etc/squid/passwd testuser
It will prompt you for the password twice.
If you open the created file, you will see the user and the hashed password.
Change the squid configuration to tell it about the authentication that it should use.
Add the following lines below the ACL ports and nowhere else to enable authentication:
.... auth_param basic program /usr/lib64/squid/basic_ncsa_auth /etc/squid/passwd auth_param basic children 5 auth_param basic realm Squid Basic Authentication auth_param basic credentialsttl 3 hours acl auth_users proxy_auth REQUIRED http_access allow auth_users
sudo systemctl restart squid
Try to open the browser again.
It will prompt you for the username and the password.