To secure all subdomains of your domain, you will need to get a wildcard certificate. Check out Wildcard SSLs offered by Namecheap.
The procedure of activation and installation of a wildcard certificate is the same as for ordinary ones. You can find the instructions here.
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.Need help? We're always here for you.