I want to create a subdomain

Creating a subdomain is done through the DNS Manager as well. To do it, you would create an A record with the subdomain's hostname. Make sure it's pointed at the IP address of the correct server. The link below gives a visual example of how this is done:

Configure Subdomains

If you're setting up an SSL Certificate for that subdomain, you have to decide which type you want to install. Since SSL certificates are normally bound to a single domain, the can't be used on other domain names which includes subdomains. Many of the users I've worked with have gone with what is called a wildcard certificate. You can read more about that in the link that follows:

Connecting SSL to a Subdomain