To secure all subdomains of your domain, you will need to get a wildcard certificate. At ZTABOX, you can purchase PositiveSSL WildCard, EssentialSSL Wildcard and PremiumSSL Wildcard certificates.
The procedure of activation and installation of a wildcard certificate is the same as for ordinary ones.
In this article, you can check more details about how the domain name should be specified for a wildcard certificate and which subdomains will be secured.