Main form in Secure mode

[Comograma]

Are you able to call your application after installing it on your server?

I mean, if you install it on your server, are you able to call it normally via http, like http://yourdomain.com using the browser?

Yes I am. Note that this Windows Server (2019) is not a WebServer, so it is not a public server exposed to the internet.
So, what I guess that's happening is that the IWCertificateManager is not able to ping my Http.sys application (https://sub.mydomain.pt/MyApp).
What I have to communicate to the server (it's a firm server and only workers have access to it) is a DynDns.
How can I, in these cases, get a Let's Encrypt certificate?