You can use Softaculous to CLONE your existing working site into another subdomain
here's the info how:
https://www.softaculous.com/docs/enduser/clone/It will NOT mess up your existing site. It will make a different db associated with the new url.
It will use the same admin logon creds (you can change them later)
Depending on your settings you may have to change the URL address within the admin settings of the new location , at least for the SSL field.
You many have to update the htaccess rewrite notification for the subdomain.
I don't know what version of Softaculous you have access to and they all act a bit differently as to getting the settings updated.
If you mess it up, you can safely delete the cloned site without affecting your working site.
Regards
The Support Team at WHY2