I'm glad to help you add a purchase number (PO number) to the form, faleman.

Adding a PO number to the purchase form is pretty simple. You simply need to add a custom field for this information.

Let me show you how:

1. Go to Settings ⚙, then select Custom fields under Lists.
2. Click Add field.
3. Enter the name of the field you want to add or select PO Number from the suggestions.
4. Choose a data type.
5. Mark Purchase order from the Select forms section
   
6. Click Save.
7. You can also include this field when your print the form. Simply turn on the Print on form option.
   

Once done, create a purchase order or open the existing recurring entry. You'll see the PO Number field on the the form. Here's a screenshot for your visual guide.

I'm adding this link to manage the workflow of your purchase orders: Create and send purchase orders in QuickBooks Online.

Keep me posted if you have additional questions about adding a PO number. I'm always glad to help you.