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 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'.