Thanks for chiming in this thread, jcorr951.

You can add your own custom fields on invoices and other sales forms if you are using QuickBooks Online Plus, Advanced, or Essentials.

Here's how can can create a new custom field: 

1. Go to the Gear > Account and Settings.
2. Click Sales on the left panel.
3. Press the Sales form content section to see all options.
4. Enter a Name into each dialogue box beneath the heading Custom fields and select whether you want the field to be internal (only displayed in QuickBooks), or public (displayed in QuickBooks and displayed/printed on sales forms).
5. Hit Save and then Done. These fields will now appear on your sales forms.

Please see this sample screenshot for a visual guide:

I'm also adding this article for more information: How to add custom fields to invoices,

Please feel free to reach out to me if you have other questions. I'm always here to help. Take care!