@SalvadorS
If I am not mistaken, setting DNS to
<domain>.<tld> CNAME <domain>.<tld>.cdn.cloudflare.net (AND)
www.<domain>.<tld> CNAME
www.<domain>.<tld>.cdn.cloudflare.net
should do the trick.
However, you can expect some problems to occur, since the Plesk Cloudflare extension (i.e. the ServerShield extension) actually manages DNS entries for CloudFlare.
If the extension is not coded to do anything with the <domain>.<tld> records, then it can be the case that CloudFlare still does not work on <domain>.<tld>.
I was investigating this and other issues with the Cloudflare extension in a recent past, but I had to give some priority to other tasks.
It was already concluded from that investigation that is better to activate Cloudflare on a site by
a) using the (lightning fast) Cloudflare DNS system, for all DNS records concerning the domain, for which you want to activate CloudFlare,
b) adding the nameservers of Cloudflare in
- the DNS records of Plesk, if Plesk is a primary nameserver (or)
- the DNS management system of the domain registrar, if Plesk is not a primary nameserver
and manage DNS records via the CloudFlare panel.
The main advantage of the before mentioned approach, besides having a work-around for the issues with the Plesk extension, is that one can activate or deactivate CloudFlare instantly, on every subdomain (with the click of a button).
Note that any CNAME record is correctly handled by CloudFlare, if you use this approach.
Finally, note that if you really want to use CloudFlare to it´s full potential, then the approach, described in points a and b, should be preferred.
Hope the above helps.
Regards....