If you ever come across the need to add an alias name to your existing subdomain, that is hosted on a Plesk server; you landed in the right place to learn how to do it!
From your Plesk Control Panel, navigate to Domains and then to the sub.domain.you.need. You should find an option called Apache 7nginx Settings:
You should now be able to see the additional Directives for HTTP, HTTPS and Nginx:
Here, you should add your desired names.
For Additional directives for HTTP, add a line like the following example:
For Additional directives for HTTPS, add a line similar to the above.
For Additional nginx directives, the format is:
An that is it. I hope you can make use of this information.