Add Order Number Automatically

I'm creating an order form in excel. I would like to add a control there where when it's triggered, excel would automatically assign a number that hasn't been used before and save the file using that number. It's like an order number generator.

For example, let's assume I'm entering a purchase order in excel. After I'm done doing that, I click on the "SAVE" control button. This will assign the number following the last saved order and save the file in a location I specify in this format: filename POnumber.