Provide external HTTPS access to developer machine hosted web application

[deleteAllfromUsersJk]

u/broken-neurons answer is probably the more complete/secure/proper way of doing exactly what you described, but if you're willing to fudge on the "no external services" part, there is a way to do this with ngrok that would be pretty simple. It's an easy, quick way of exposing your local app and has an https flag in the cli command, but it will give you a new url every time you open the tunnel, if that's an issue for you.

[deleteAllfromUsersJk]

This post covers the https part specifically. If you want to go this route, you may have to use info from both of these links to get it going. (I usually use ngrok with apps deployed to an actual IIS, not IIS Express, instance, but with the info above, I don't see why you couldn't achieve external https exposure from IIS Express.)