That's not a keyboard shortcut for FileMaker at all. Tab and Shift Tab will move you from field to field in both a layout as a whole and within a portal row.
ctrl-up/down arrow does move you from record to record, but that won't help you with a portal.
You might be able to set up onObjectKeystroke to support using the arrow keys like this, but then you lose the ability to use them to move the cursor around inside the current field.
i tried it out and cant make it to work in the portal fields. i guess the tab keys are ok for now.
Thanks for the help PhilModJunk!