XFi port forwarding to a different port

Is it not possible to set up a port foward on an XFi router that sends an external port to a different internal port?  For instance, 9989 -> 80?

I have a printer/scanner that I want to control remotely, and the device's web admin interface is on port 80.  For security reasons, I don't want external port 80 open.  And in fact, when I tried forwarding port 80, it seems not to have worked, because the page at internet.xfinity.com/network/advanced-settings/portforwarding shows no existing forwards, just an "Add" button.

I tried accessing the router directly at 10.0.0.1, but the port forwarding section just contains a link to xfinity.com/myxfi - presumably that interface is disabled because it's an XFi router.