We have some internal applications that requre custom ports in the URL, i.e. https://webserver.company.com:8787. When using the app proxy for these types of sites, things work as expected only you're external:

External user clicks the app in the myapps portal... They're provided with the external URL and the app proxy then grabs the website based on the internal URL and presents the pages to the user. GREAT! All works as intended.

However, the issue comes when you're an internal user accessing the same myapps portal and clicking the same application. The app proxy determines you're internal, but still hands you the External URL (without the custom port). Since you're sent directly to the internal server using this URL, the webpage fails.

The app proxy obviously knows when you're internal, so in those cases it should provide the Internal URL instead of the external one. This would prevent site failures and allows the custom port to be used properly when the users are internal or external.

The only workaround (per MSFT Sr. Engineer) is to create two apps in the portal, one labeled "INTERNAL" with its homepage URL editied to include the custom port, and the other app labeled "EXTERNAL" and setup in the normal fashion.

In his own words, "this is clunky, not clean, and will confuse the users, but unfortunately this is our only option at this point in time."