WWW subdomain for existing branch subdomain (for viewers who always type www ...)

I have a custom subdomain
subdomain.mydomain.com
that displays the content of a branch subdomain of
mysite.netlify.app
(NETLIFY DNS).

There are people viewing this subdomain who are not so technically inclined.
Therefore, I want an alias
www.subdomain.mydomain.com

But NETLIFY and CNAME or ALIAS do not seem to work together.
CNAME or ALIAS from
[www.]anything.mydomain.com
to
subdomain--mysite.netlify.app
or
subdomain.mydomain.com
not possible when branch subdomains (NETLIFY DNS) are set up?
Solution?

Thanks in advance!

Could you share the domain or the site name?