You'll have the option to "Add PO Number" when you're viewing a project's Contract tab.
Once your clients have reviewed their quote and signed the contract online, they'll see a link below the Make Payment button to add a PO Number.
After clicking that option they'll see a form field to add their PO to the project:
Once the client saves the PO number, the PO number will now show up on the invoice and the client is now able to print the invoice with the PO number attached to it.
Sales users linked to this project will also receive an e-mail notification that the PO number has been added with a copy of the signed contract (w/ PO# on it) attached.
When a PO # is set for a signed contract, whether added by client or vendor, a copy of that signed contract with the PO # is generated and attached as a file to the project.