Manipulating user agents in squid

[xize]

Hello,

So I was trying to follow this tutorial:
https://www.suse.com/communities/blog/squid-proxy-anonymous-browsing/

then I saw there was a way to hide the user agent from people their browser I thought it could be usefull against tracking.
but now I wonder how could I use this code:

   header_access User-Agent deny all
   header_replace User-Agent SecretBrowser/5.0 (iPhone; U; Commodore64; en)

i've tried to use the code inside the advanced configuration under the textboxes on the start page of squid in pfsense:
under "Custom Options (Before Auth)" and in "Custom Options (After Auth)" but it didn't seemed to work.

Is there a way to get to the squid.conf?

I had readed in some topics that the config was either hardcoded or was not persistent, how can I make these changes persistent?

Thanks!