If you have a valid SSL certificate for yourdomainname.com, it will be valid for any URLs that begin with either https://www.yourdomainname.com OR https://yourdomainname.com
Therefore, you can install WordPress in the root (https://yourdomainname.com) or in a sub-folder (https://yourdomainname.com/wpinstall
However, you will not be able to use sub-domains like https://wpinstall.yourdomainname.com (unless you spend about $350 for a "wild-card" SSL Cert).
Be sure to change the WordPress General Settings to have https://yourdomainname.com (or https://yourdomainname.com/wpinstall) for the WordPRess URL and Site URL.
IMPORTANT: When you change the WordPress settings as mentioned above, to log in to WordPress you will have to type the entire URL including the "https://" in your browser like this (without the quotes):
"https://yourdomainname.com/wp-admin" (or "https://yourdomainname.com/wpinstall/wp-admin")
The reason you have to actually type "https://" at the beginning is that if you type only the domain, your browser will automatically take you to the "http://" un-secure URL; then WordPress re-directs to "https://". When WP does the re-direct, there is a bug in WP that won't let you log-in (it will just keep displaying the log-in screen over and over again); so it seems like there is an issue with the SSL, but there is not.