Show content from subdomain.mydomain.com to clientdomain.com without migrating the files

I need to show the content from a subdomain to a domain without actually migrating the files.

Here's the explanation of what I do:

I generate websites for clients on subdomains. This is done because most of the time the client doesn't yet own a domain.

Then, when the client buys a domain, I want to show the content that was built on the subdomain on his domain. I need to to this without actually migrating the files. This should be done in CloudFlare or somethin similar.

Notes:
- the client should be able to do this on his own, I don't want to ask for credentials to manage his domain
- it should be done in the easiest and simplest way
- we could use Cloudflare and show the client how to configure his domain through Cloudflare (here we can use workers, rules, dns, etc)
- I have access to the hosting server completely through vultr.com and I'm using Plesk (I can also use cyberpanel) to manage the server.

TLDR: I am trying to show the subdomain content on client's domain. I believe we can use Cloudflare Workers for reverse proxy setup.