How do I install a modern Add-in using a URL?

In the interest of security, Microsoft no longer supports installing an add-in directly from a URL, but only from a downloaded file.  To do this:

1. Go to https://aka.ms/olksideload to open the Add-Ins for Outlook dialog.
2. Select 'My add-ins'.
3. In the 'Custom Addins' section, select 'Add a custom add-in', then choose 'Add from file'.
4. Browse to the file you downloaded and then click 'Open'.