How to enable wildcard SSL for subdomains?

I am using this plugin
https://codecanyon.net/item/subdomain-pro/15345048
that makes wordpress pages in a subdomain like
page.domain.com (instead of Default title | Domain.com)

but thing is the ssl is not enabled for it.

How do I enable the wildcard SSL for subdomains?
I mean the free SSL letsencrypt

Thank you