You can get the portal URL for a customer by looking at their record (click on their entry on the Contacts List screen.) There is a field for the Portal that has a link to "Share" which will bring up a pop-up window displaying the portal URL for that customer. You can copy the URL from the pop-up window.
The message "Customize -> Portals -> Get sign-in page web address..." refers to the Classic version of Method only.
Mary