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:

/etc/squid/squid.conf

....
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.