First, access your cPanel hosting via the Login/Access button in the Manage cPanel Hosting section.  

When in the cPanel dashboard, locate the Domains option under the main Domains section:

In the domain section you will have 2 main option for adding secondary domains:  
 
Either adding an existing, registered domain, or adding a temporary ( non existing) domain that can server as a staging site.  
 

 

When adding a new, registered domain, please make sure to untick the “Share document root” option so the secondary domain has it’s own directory. If left ticked, the newly added domain will share the same content as the main domain.  
 
With the way cPanel functions, when adding additional domains, a subdomain will also be added on the main domain. This can largely be ignored and go unused as it’s only a technical requirement from cPanel and does not affect the functionality of the other domain.  
 

 

After clicking the Submit button, the new domain will be added. For secondary domains added this way, the DNS will not be added automatically even if the DNS is managed by Heart, and it will need to be updated manually.