How do I add a client purchase order onto my invoice, ie how do I show this on the invoice is there a custom column?

In new QBO, they’ve up to three custom fields selections available to customize invoice layout.
You can pick one field for Client Purchase Order Number.

From the Home page screen

• Select the Company Gear (located top upper corner)
• Select “Company Settings” under Settings header
• Select “Sales” under Settings”
• Select “Sales Form Content”
• Select “Custom Fields”
• This is where you add Client PO No.
• Save and Done
• See screenshot photo below for reference