That's why, when you activate HTTPS in the Admin-panel, you see 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 Let's Encrypt. Just go to Admin-panel > Internet, and you'll see the dedicated box:

If you don't have a domain, you can opt for a basic (self-signed) certificate 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: