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