You could set an OnObjectKeystroke trigger to look for Char (13) or Char (10) and then write a script that goes to the next portal row, then to the same field. Should work.
One issue about the keystroke trigger is that it triggers on EVERY keystroke. Depending on your network performance you can see a bit of a lag. I had one system where it worked fine on the LAN but as soon as we started testing over a WAN the was a noticable hiccup on text entry. You also have to watch for fields where the Enter key is a valid key within the field.
Filemaker has a built-in shortcut to do this, 'Ctrl-Down Arrow' (on a Mac). Also 'Ctrl-Up Arrow' moves up a record.
Perhaps you can use a keystroke mapping utility to send a Ctrl-Down Arrow when you click 'Enter'?
Best wishes - Alan Stirling, London UK.
FM can handle this natively. If you turn off entry to the other fields except the one you want to work in ... and then go to Inspector > Data tab and then under 'go to next object' check both Return and Enter, no trigger will be needed. Might this work for you? FM moves to the next row in a portal by itself. :^)