https://outlook.office.com/mail/inclientstore or its alias https://aka.ms/olksideload are the two URLs which open catalog Add-Ins for Outlook just over the standard e-mail view:
But the above hyperlink appears to be “special knowledge” because in Outlook web UI, I was unable to find steps to get to that window. Is it possible to open exactly that window without the knowledge of the above link?
(I know there is a way to open similar list via More Apps > Add Apps on the left panel, but it opens different UI with somewhat different list.)
