Thank you for raising this out to us, vallomedia. Let me share with you how to add a custom fields on sales forms and purchase order. 

1. Go to Gear icon and select Custom fields.
2. Select Add field.
3. Enter a name in the Name field.
4. Tick the All Sales forms or Purchase Order box. Or, tick both.
5. To show the custom field on printed and delivered forms, turn on Print on form. Otherwise, it’ll only appear in QuickBooks Online.
6. Select Save.

Please note that you can only create 3 custom fields which is available in QuickBooks Online Essentials and Plus. 

Feel free to message us back if you have any other questions. I'm here to help. Have a great day!