Adding a purchase order (PO) number to an invoice

How to add a purchase order (PO) number to an invoice for a client. 

1. Steps to adding a client PO number to an invoice. 

2. Find the booking. Click here to learn how to do this. 
3. When on the booking page, scroll to the invoicing and payments section.
4. Add the PO number to the PO# section of the invoice. 
5. Press the update / save button.
6.