Alert pulling data from input sheet... not sure its even possible!

Alert pulling data from input sheet... not sure its even possible!

I'm creating a purchase order form.  I've imported the PO numbers to be assigned to Column 1 of the data sheet.  The other columns are blank.  What I would like is for the data from the form to be saved in the next "available" PO number row (if that makes sense) AND for an alert to come up and tell the person filling in the form what the assigned PO number is... does that make sense?!