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.

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.

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