No. You cannot retrieve the URL. I'm sorry I don't have better news, but you will need to track down the original calendar link you received.
Web applications have unique advantages and disadvantages over their desktop counterparts. A desktop app wears many hats while web apps are typically more focused. There is little data stored locally for Outlook on the Web (OWA) while Outlook stores gigabytes upon gigabytes of information locally which contributes to a more comprehensive offline experience. Even if you configure OWA with offline viewing enabled it is extraordinarily limited in what it includes.
Outlook on the Web (OWA) and Outlook differ significantly in the way they manage Internet calendars:
In Outlook
If you add an Internet calendar in Outlook the configuration and storage are done locally. If your email account is Exchange-based, that Internet calendar's configuration info will be shared with other PCs that are also running Outlook and they in turn will manage the subscription and create their own local data store. Internet calendars set up in this way will not be displayed in the same account in OWA.
Outlook on the Web
If you add an Internet calendar in OWA, the server completely manages the calendar subscription and all updates (roughly every 4 hours). A calendar added in this way will also show up in the same account in Outlook however it will not have the properties of an Internet calendar. The properties will be more akin to an Exchange calendar with some missing options.
There are always competing features and finite resources in development which may have contributed to this gap in functionality, or it may simply be that there was no perceived need to retrieve the original URL once the calendar was configured server-side. Many, more basic features, that people rely on daily from Microsoft's desktop apps haven't, or haven't yet, been incorporated in their web counterparts.
