All knowledgebases hosted through Elevio (with or without a custom domain) come with TLS certificates by default. We use Let's Encrypt as a certificate authority for all our customers. 

Using your elevio subdomain

If you don't need a custom domain there is nothing that needs to be done, TLS will be available right away.

Using a custom domain

After you have configured your custom domain on the settings page a TLS certificate will automatically be issued during the first request to your custom domain. This process will take about 10 seconds, please do not refresh the page. Once the certificate is issued you will be forwarded to your knowledgebase.

If you have any CAA records configured with your hosting provider you need to ensure that you add a record for Let's Encrypt before you configure a custom domain. You can follow this guide: https://letsencrypt.org/docs/caa/

Can I host my knowledge base in my existing main domain in a folder, like mysite.com/knowledge?

No. The knowledge base needs to operate on its own full domain or subdomain, it cannot run as part of an existing site.