Hello @Duncan ,
All you need to do is mention the port number in the Internal Url field along with the URL as mentioned below:
Internal Url : http://your_webapp.private_domain.local:8080/
External Url : http://your_webapp.public_domain.com/ or https://your_webapp-your_tenant.msappproxy.net/
I have tested with a website hosted with port 8080 in the on-prem server and with above configuration, I am able to access the web application via app proxy without any issues.
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