So, you want to secure a parked domain in your cPanel with an SSL Certificate and make it accessible via secure connection, e.g. https://example_alias_domain.com.
It is not actually possible to install a separate certificate for a parked domain. The parked domain and the main domain can only have one SSL certificate assigned to both.
Keep in mind that "www" subdomains will not be added to such a certificate
automatically. Thus, an additional SAN has to be purchased to cover 4
addresses at once (2 bare domain names and 2 "www" subdomains).
Note: It is not possible to secure a parked / alias domain using the NamecheapSSL plugin.