First of all, you need to get a certificate for your domain, you can use this service for example: https://www.sslforfree.com/ , choose the DNS TXT record option.
Select the manual configuration mode, set the records in your domain zone, which the certificate issuer will provide to you, the domain zone is to be changed within 2 hours, after that click on the receive button and get the certificate for your domain from the issuer's end.
The received data should be: a secret key, a certificate, an intermediate certificate.
In order to receive more detailed support, please send a message into the support team from the Control Panel of your website.
We recommend that you contact the support team from the Control Panel of your website. Also, if you want to know how to use the Letsencrypt service, it's better if you check this material from their website https://letsencrypt.org/getting-started/