How to connect your new site to your Merchant Facility
This step is only required if the Base Site URL on the Merchant Facility is blank or there was an issue with the Site Testing.
1.1. Select the Cog symbol on the right hand side of your Salesforce home screen. (In Classic, click 'Setup' on the top right hand side.)
Copy shortcut - Highlight - right click and select copy.
Make sure you choose the SECURE URL (not shown below). A secure URL will begin with https// (note the "s" in "https"). The URL should also contain ...secure.force.com.. in it (prior to Enhanced Domains Enablement), or ....salesforce-sites.com... (post Enhanced Domains Enablement [Jan 2023])
NOTE the Secure option does not show for sandbox and developer orgs, but will in production.