Outlook for Mac - archive single email with a keyboard shortcut

Can be done via Keyboard Shortcuts under System Preferences

Manually move the email once to the folder that you want, say Archives by right-clicking the email and choose it or using Shift Cmd M. After that Archives will start showing as one of the options when you try to another move email.

Once you’ve moved the message the folder becomes a recent item under the Move menu button in Outlook and a keyboard shortcut can now be assigned to it.

Open Keyboard Shortcuts under System Preferences and assign your favourite keyboard shortcut e.g. Control-Option-A to move messages to this folder (Archives).

This isn't exactly the question you asked, but Ctrl-e is the keyboard shortcut for archiving a message (at least as of Outlook for Mac version 15.36).