Glad to see you here in the Community, user58666.

I can help you with adding purchase order numbers on the delivery note that you'll be creating.

You can customise the invoice by adding another field and identify it as Purchase Order. Once you've created a Delivery Note out of it, purchase order number will now be included in the delivery note. 

Here's how to customise the invoice:

1. Go to your Invoices tab and create a New invoice. 
2. Click the Gear icon on the upper right corner in your Invoice window.
3. On the Fields, click Add another field. 
4. Click the box beside it and indicate it as Purchase Order. 

Purchase Order is now added on your invoice. I've also added screenshots below for your reference. 

Check these articles for more information:

• How to create a delivery note or delivery note 
• How to add custom fields to invoices

That should do it. Let me know if you need further assistance in creating a delivery note. I'm always here to assist. Have a great rest of the day.