We have simple ways on how to do this in QuickBooks Online (QBO) and I'll show you how, 

precept.

You'll have to manually enter the Purchase Order (PO) number on your estimates if you want it referenced on your sales form. Follow these steps on how to do add a custom field:

1. Click the Gear icon, then select Custom Form Styles.
2. Click the drop-down beside the New style button and choose Estimates. 
   
3. Go to the Content tab, then click anywhere on the first part of your estimate form.
4. Click Custom Field, then add Purchase Order on the field and tick the box. 
5. Click Done.
   
6. Once done, open the estimates then click Customize so you'll be able to use the template and for the Purchase Order number field to show.

Then, you can now convert estimates to invoice. And then, manually re-enter the PO number on your invoice. You can still follow the same steps above, but you'll just have to select invoice instead of estimates.

Finally, you can run Purchase Order reports in QuickBooks Online.

You'll also want to browse QBO help articles in case you need further information about the product.

If there's anything else that you need with sales forms, let me know so I can assist you.