Panel has an easy option to set up Domain Aliases, which assigns the primary domain of your cPanel an Alias domain.
Look for the "Aliases" option under the Domains panel:
Important: If you're mapping multiple subdomains, check the article about Mapping with Subdomains.
Then, create a new Alias by typing in your Domain:
Ensure the document root is the same as your primary WordPress installation.
Here's a video about how to configure Aliases in cPanel: