I have example.com hosted on cloudflare. How do I add the "www" subdomain www.example.com?

[barren vapor]

I can access example.com through the browser but www.example.com does not work. I tried adding a CNAME.txt file to the root of the Astro project directory containing the following lines:
;; CNAME Records
example.com. 1 IN CNAME example.pages.dev.
blog.example.com. 1 IN CNAME example.com.
www.example.com. 1 IN CNAME example.com.

In cloudflare I have the following setup:
"www.example.com is an alias of example.com and has its traffic proxied through Cloudflare."

[drifting solstice]

You’ll probably need to edit CNAME settings directly within Cloudflare, or wherever your domain is hosted.