Hm yeah I don't think this is doable via a form; at that point the record data doesn't exist so you can't filter by anything. If you've just got two linked fields and a record that already exists this works fine though:
===
To get around this you can try creating a non-form Interface page instead (check out this thread), but you'd need to create a process for them to create the record first before going into this page, which might be awkward UX wise