How to create incremental number in forms for Purchase Order PO

Hello, 

We are moving from G-suit to office 365, and we are going to use forms. I have created a couple of forms and they work fine. Now I needed to add the incremental number when a staff creates a new PO a new number is generated Automatic, For example: 

Name: Test Date 13/11/2018 Department: Sales  Item to purchase: PC       PO : 
once the user submits the form the PO is created.