I realize that I'm out of my league in this discussion but

my AddOn domain ZenCart configuration (setup in Cpanel as AddOn domain not Subdomain) are just as though they were the main domain within my account

http://MyMainDomain.com

http://MyAddOnDomain.com

Code:
define('HTTP_SERVER', 'http://myaddondomain.com');
  define('HTTPS_SERVER', 'https://myaddondomain.com');

  // Use secure webserver for checkout procedure?
  define('ENABLE_SSL', 'false');

// NOTE: be sure to leave the trailing '/' at the end of these lines if you make changes!
// * DIR_WS_* = Webserver directories (virtual/URL)
  // these paths are relative to top of your webspace ... (ie: under the public_html or httpdocs folder)
  define('DIR_WS_CATALOG', '/');
  define('DIR_WS_HTTPS_CATALOG', '/');