Hi @Oli in Plymouth UK
The standard way to add a PO to a Sales Invoice is to use a Custom Field.
I can't tell which version of QB you're using but that screenshot looks the same as mine (QuickBooks Essentials).
Create Invoice > Click the Cog symbol > Custom Fields
or click https://app.qbo.intuit.com/app/customfields when logged in.
You'll probably need to add the new fields to your invoice template.
Hope this helps.