We don't have any IIS guides for a proxy, but this guide looks like it would work: https://docs.microsoft.com/en-us/iis/extensions/url-rewrite-module/reverse-proxy-with-url-rewrite-v2-and-application-request-routing

The key is that version 1.19.x of FusionAuth is completely stateless so the proxy can round-robin and no session pinning is required. If you are using a version of FusionAuth before 1.19, you'll need to pin your session to ensure that you can log into the administrative interface.