When you map a custom domain, that makes your Squarespace site available at that custom domain URL. However, the site still remains available at your Squarespace.com URL as well.
When you copy the page URL from the address bar in your browser, you are copying the Full Squarespace URL for the page. Instead of using that full URL, you can remove the root portion of the URL and use only the shorter, Site-relative portion. This will allow the system to automatically display the root domain based on the root domain the visitor is currently using. For example, to link to a page on your site here:
Instead of adding the Full URL like this:
Add the Site-relative URL like this:
Note: System generated links, such as those created when you add a page to your site, will automatically use your Site-relative URL.