How to change SSL port for https with an ASP.NET MVC application deployed on IIS in an Azure VM?

I've inherited an older ASP.NET MVC project deployed to an Azure VM, and I'd like to make a duplicate website on the VM so I don't test by deploying to production.

Ideally I'd like to take what's there now, duplicate the files, and set that up as a new website before deploying over top of it. (We can argue the merits of this approach later. ;^D)

Aside: I initially thought I'd make a subdomain which would allow me to keep all the settings the same, namely ports, and was following the instructions in this video. When I tried that, however, I found that DNS (and, therefore, DNS Manager) was not an option in Server Manager on the VM. Assuming I needed to set an equivalent setting in Azure, I opened the page for DNS Zones in the Azure account (as described here on learn.microsoft), but there were no zones listed there either.

I took that to mean the DNS records are at GoDaddy or Hover or something (the previous contractor did not leave documents on that -- I'm sure I could find out from the client if that's the easiest route), guessed that IIS can't handle subdomains downstream for some reason (though I've done it before in nodejs/express) and, instead...

I've tried standing up a beta site by copying over files, adding the IUSR with read permissions to the new duplicate directory, and changing ports. http I set to 81 and https I set to 8443.

I can get the initial home page to work on :81 using a browser local to the box (since I wanted to get it working before dealing with firewall settings), but it forwards to :443 once it's using https.

How can I convince the pages at :81 to use :8443 instead of :443 that sends them back to the original website?

I'd hoped I could edit appsettings.json, but this seems to be an older project with no appsettings file. There is a Web.config page, but nothing there about SSL. The only places I see 443 anywhere in the app are:

• a * - WebDeploy.pubxml file for its MSDeployServiceURL property.
• In jQuery. 😕

Where should I tell the site that it needs to sub 8443 for 443?

To be clear, I'm not looking for IIS Express or some setting in Visual Studio. I'm trying to figure out where it's set from the server's point of view. If there's a Visual Studio UI to set this, great, but what's the end result of that configuration change? What's the operative change on the server?