Custom HTTPS port not showing up in URL

I am trying to publish an internal web server over the WAF. The setup is as follows:

• Internal name: server.company.local , reachable via Port 8089.
• External name: server.company.com (I have a public DNS record pointing to my static public ip).
• I've created a real webserver with the internal hostname / IP as the host, Type: HTTP and port 8089.
• I've created a virtual webserver with type: 'HTTPS & Redirect', port 8089 and a valid letsencrypt certificate for the public name.

I can access the website and login, so that worked. The first problem was that any link on the website pointed to the internal hostname. I resolved this by activating "Pass host header" in the virtual webserver. Now the links point to the correct hostname but the custom port (8089) is missing so when I click on a link it tries to establish the connection via port 443. What am I missing?