Reference a custom field from another custom field

I have a custom page in IFS Applications 10. There is a custom field "Part no." on it that is a reference field. It selects from an LOV of part numbers.
There is a second custom field "Location" alongside it that also selects from an LOV (of part locations) and I want to filter that field to only display locations that the part that is in the "Part no." custom field is in.
However it isn't until these two entries are completed that the record on the custom page can be saved. So I need to be able to grab the value of what is in the Part No. field (before the record is saved) and use that as a filter on the Location custom field.
Is this possible? See screen cap attached.