Add on domains are technically different domains that point to another "main" domain server.
From a Cloudflare perspective, however, the domains are looked at as unique entities, which means that you would have to add each domain separately.
Cloudflare treats each of the following as a separate domain:
Each needs to be added separately to your Cloudflare account, and may have a separate plan level, from Free to Enterprise. There's no limit on the number of domains in a Cloudflare account. If you have hundreds or thousands, as some customers do, you'll want to know how to use the API v4.
Note: all subdomains are included, so these would all be one domain for Cloudflare plans: