First, make sure you have connected the domain name as an addon domain in your cPanel. If not, follow these instructions: https://www.hostpapa.co.uk/knowledgebase/kb/how-to-set-up-an-addon-domain/
As you set up your addon domain, the system will create a folder for you. By default, it will name the folder after your addon domain, but without the “www.” For example, if you add a domain called www.addondomain.com, the folder will be called /addondomain.com. Upload your files to this folder.
You may upload files to an addon domain using your current FTP account or you may set up a new one.
More details on addon domain FTP settings: https://www.hostpapa.co.uk/knowledgebase/kb/ftp-settings-for-addon-domains/
What’s the difference between an addon domain, a parked domain, and a subdomain?