Trying to understand reverse-proxying using nginx and Windows IIS

So we had a contractor set up nginx on an Azure-based Windows 2016 server which we wound up for the purpose. Now I'm trying to get my head around how to use it (and, ultimately, to fold back into our existing servers.)

The goal is to be able to have an IFRAME on our site with a 3rd party's site in it. Right now that doesn't work because of a "same-origin" restriction. We've asked the 3rd party to add use to the list of permitted servers but here we are three years later and no add.

So we thought we'd try the reverse-proxy approach. Now we have nginx on a server with the following nginx.conf (redacted):

    server {
        listen       80;
        server_name  localhost;

        location / {
            proxy_pass https://www.3rdparty.com/;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host 3rdparty.com;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_cache_bypass $http_upgrade;
        }

The contractor left IIS's binding at ":8081" for the default website.

How do we use this? What has to change in IIS, if anything, and in the nginx.conf?