You need to change nginx conf files. Something like
location / {
return 302 https://$http_host/yunohost/admin;
}
to
location / { return 302 https://$http_host/; }
in /etc/nginx/conf.d/yunohost_admin.conf file, which seems to be used when you go on a subdomain not available on your yunohost instance.
You’ll have a warning in your browser (which search an available TLS certificate for the nonexisting domain), and nothing is showed.
It’s a way, it needs some improvment (to redirect to the page you want.