Proxy --> Block all websites except those whitelisted

[OPNFan]

Hi All

New to OPNSense and loving it so far!  From a proxy point of view, how do I block all websites on the internet, except for white-listed ones?

From the post below it appears that one can do this by editing the squid.conf file:
https://forum.pfsense.org/index.php?topic=25728.0

I'm hesitant to do that without understanding the repercussions for the WebGUI.

Is there any obvious way I'm just not seeing?  Like setting blacklist to ALL/$ (long shot there)?

Any help would be appreciated.