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.

The parked domain and the main domain can only have one SSL certificate assigned to both.

Therefore, the most convenient way to secure your main domain and the alias is to use a Multi-Domain Certificate with both included. It can be installed in cPanel for the main and the alias domains one by one following the usual SSL Installation Guide.

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).

There's also a convenient workaround to install an SSL certificate and activate it for your alias with the Web-based automation option. You can use this guide as the reference.

Note: It is not possible to secure a parked / alias domain using the NamecheapSSL plugin.