First lets set the web version of Outlook like a desktop app
- In Microsoft Edge, sign in to your Outlook on the web or Outlook.com account.
- Select Settings and more
at the top of the window.
- Select Apps > Install this site as an app.
Microsoft is now advertising that Outlook can be designated as the default email client.
The feature makes use of the “URL protocol handler registration for PWAs”.
(This page wants to install a service handler.)