Whenever you add a new site to your wao.io dashboard, the preview version of that site, which is a subdomain of wao.io's wao.zone, is also running on https. Here's what you need to do if you'd like to preview your website using https.
If you're already using an ssl certificate for your website, you don't need to change the settings. Just click on the link under "Frontend" to open the wao.io preview of your website in your browser using the Let's Encrypt ssl certificate. If your website automatically redirects to https, so will the preview. If it doesn't and you'd like to preview your website with ssl always on, go to Settings, switch to the Optimizations tab, and enable "Redirect HTTP URLs to HTTPS" at the bottom of the list.
If you're not using an ssl certificate for your website, you only need to make some adjustments to experience SSL on your preview website. Open Settings and head to the "Backend Protocols" dropdown. By default, this is set to "talk HTTP & HTTPS to my origin domain". Change it to "talk only HTTP to my origin domain". Now, switch to the Optimizations tab and enable "Rewrite HTTP URLs to HTTPS", which is the second last option of the list. To automatically switch to https for your preview, also enable "Redirect HTTP URLs to HTTPS" at the bottom of the list.
In both cases, keep in mind that wao.io offers a free SSL certificate for your website using Let's Encrypt once you switch from Preview Mode to Live Mode.