Having the option to customize your invoices to include fields such as a PO# is essential. QuickBooks Online has your back, and offers a quick and simple way to add custom fields to your invoices. I'd be happy to explain how to do it.
If you use QuickBooks Online Plus, or Essentials, you can create your own custom fields on invoices, sales receipts, refund receipts and credit memos.
Follow these steps to add a Purchase Order # field:
1. Click the Gear icon ⚙️.
2. Select Account and Settings.
3. Select Sales, then go to the Sales form content section.
4. Select the pencil icon in that section.
5. Enter a name into each dialogue box beneath the heading Custom fields and select whether you want the field to be internal (only displayed in QuickBooks), or public (displayed in QuickBooks and displayed/printed on sales forms).
6. Select Save and then Done. These fields will now appear on your sales forms.