In fact, Yunohost will automatically define a hostname when you run the postinstall using the domain you chose … Not 100% sure how it works, I think if you define “subdomain.my-domain.tld” it will use subdomain as the hostname … But it also kinda depend on what exactly you’re talking about, because there’s the hostname, the short hostname and the pretty hostname …
From what I know, the hostname is merely “cosmetic” ?