To install a custom SSL certificate or Let's Encrypt certificate to your subdomain, you will have to go to your network view. 

Once there, find the domain which hosts the subdomain. Then click on "Actions" > "View Subdomains" and there you can see your domain's subdomains. Click on "SSL Certificate" button to view your subdomain's SSL certificate information. If you don't have an SSL certificate installed click on the link to open the installer.

SSL installation runs in 15 minute intervals.