How do I install the SSL certificate (activate the HTTPS secure protocol)?

You can configure the SSL certificate using the How To Set Up HTTPS On Your Website guide.

To put it briefly, you need to have two A type records that you have added while connecting the domain. They have to point to the IP addresses provided by the system.

To view the IP provided by the system, go to the Site Settings → Domain, specify the domain you need, save the changes → you will see the IP address you need.

An example of adding A type records:

In 24 hours after you have added A type records, go to the Site Settings → SEO → HTTPS Settings.

Switch the toggle to the "ON" position to generate the security certificate.

Wait for 5-30 minutes. The certificate has been connected if you see the "secure connection for this website is on" status.

If you want the website to open using only HTTPS, configure redirecting HTTP to HTTPS.