Hi lloyd_catherine_b,

Thanks for joining the thread.

The Sales tab my colleague was referring to was on the Settings page of QuickBooks Online. For you to be able to add custom fields in your sales form, you can follow the steps below.

1. Click on the Settings icon at the top right.
2. Click on Custom fields.
3. Click on Add custom fields button.
4. Enter the name of the custom field.
5. Select the form type where you want to show the field.
6. Enable print form.
7. Click on Save.

Then, create an invoice and the custom field should show on the invoice creation page.

Also you can check this link to learn more about custom fields Add custom fields to sales forms and purchase orders in QuickBooks Online.

Feel free to send a reply here if you need more help with your sales forms. I'll be around to help you!