Make dropdown field default to most recently added entry

Make dropdown field default to most recently added entry

I have a lookup field w/ a dropdown (for Customer) within an invoice-like form.  I would like the dropdown to default to the most recent entry so that the user doesn't have to search out the newest customer...  I have option enabled to add new entries on the fly (check box for "Allow users to add new entries in XX"), and would like them to populate in the dropdown after being added.

Thanks!