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Actually it is possible in cPanel but not on vPanel. Don’t know the exact reason.
You are weird.
Mostly people want a short URL for their domain.
Because more short, more higher its price. & more easy to be remembered/typed.
Technically, you can do that. All you need is Cloudflare.
You add an A Record to point to another domain, for example:
|sub2.sub1.example.com||IP of another website|
I like long, say for account login, instead of hn.fyi/account/login, i would use http://login.eu.hn.fyi
May this helps:
And subdomain of subdomain will challenge you to subdomain rate limitings, ssl limits, and etc which you need to encounter, though.