Managed firewall rules on specific host

I have a customer that uses cloudflare as a CDN and for ddos protection on their application as well as their website. Their website is wordpress so I’d like to enable the managed firewall rules to protect it further but I don’t want to affect the other hosts in the same domain. How do I enable firewall rules only for particular hosts (i. e. just www)?

You can set http.host for that rule: