It's great to see you here today, @gregorypolk4. 

Let's add a custom field to the invoice to include your client's purchase order. I'll guide you how. 

1. Go to the Gear (⚙️) icon on the upper right corner. 
2. Select Account and Settings under Your Company. 
3. Go to Sales from the left menu.
4. Click Sales form content.
5. In the Custom fields section, enter the Name, such as PO Number. 
6. Check the Internal box if you want to show the field in QBO. Choose Public if you want to show the field on customer forms.
7. Choose Save.
8. Click Done.

The screenshot below shows you the last six steps. For the detailed instructions, check out this article: How To Add Custom Fields To Invoices. 

Once done, manually enter the purchase order number in the custom field when creating an invoice. I recommend customizing the sales form before clicking Save and send to email it to your client (see the screenshot below). 

Here's what it looks like after performing the steps above: 

Once you accept the invoice payment, record it from the + New menu. It helps ensure your client's balances are up to date in the program. 

I'll be right here to help if you need further assistance. Take care always.