System: NetXMS 3.2 listening on the default port of 8080 on Windows Server 2019

Download two files:
stunnel from https://www.stunnel.org/downloads.html
nssm from http://nssm.cc/download

Install stunnel using the installer.  It will have you create a new self-signed certificate.  If you want to use a signed one, there is plenty of info out there on how to do that.  If stunnel starts, stop it using the tray icon or kill it.

Edit the stunnel config file (C:\Program Files (x86)\stunnel\config\stunnel.conf) and find the "TLS front-end to a web server" and make it look like this:

; TLS front-end to a web server
[https]
accept  = 443
connect = 8080
cert = stunnel.pem

Download and unzip nssm to c:\software\ or another non-user-specific location (e.g. c:\program files\ ).
Open an Administrative command prompt and run nssm like:

"C:\software\nssm-2.24\win64\nssm.exe" install stunnel

Browse to and select the stunnel executable ("c:\Program Files (x86)\stunnel\bin\stunnel.exe"), enter a description of "SSL for NetXMS" and click "Install Service"
Go to Services and start the service.

Browse to your NetXMS server using https://netxms.my.org/nxmc  (don't forget the nxmc).

Yes, you have to add the /nxmc at the end.

Worked great for me, hope it helps someone else who doesn't want to setup an IIS reverse proxy, etc.  YMMV.