The best solution is to set the name of your system correctly. You can use Webmin to set it up in the Network Configuration: DNS Client page, plus you need to actually make the name resolve, either via a hosts file entry or a DNS record (preferred--just setting it in hosts doesn't do anything useful for your proxy users, which is why this check exists).
If you must use the visible_hostname option you can set it in Webmin's Squid module in the Administrative Options page. The field is labeled "Visible hostname".
But, if you use visible_hostname, any error pages it generates will point to a non-existent name.