HTTPS cannot work without a "certificate" file. That's why, when you activate HTTPS in the Admin-panel, you get something like this:

If you already have a certificate, provide the certificate file (with its companion file, the private key), otherwise HFS can produce one for you, for free, thanks to Letsencrypt. Just go to Admin-panel / Internet, you'll see the dedicated box:

If you don't have a domain, you may opt for a basic certificate (self-signed), just by clicking the "make one" link, but this has the downside of showing an ugly warning to the final user. This is what the user will see: