When selecting Dropbox as file location it will save the ics file in the Dropbox' public folder. If your Dropbox has no public folder or if you would like save the file into a different folder within your Dropbox (or any other local folder for that matter), you can do so by selecting Local from the File location drop-down menu and setting the File path accordingly.
To retrieve the public link of the ics calendar file, open the Finder on your Mac and navigate to the location of the exported ics file, right-click the file and select Copy Dropbox link.
Please note: the string dl=0 at the end of the link needs to be changed to dl=1, otherwise it is not possible to subscribe the calendar.
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