Hello there, @accounting54.

Thank you for posting here in the Community. I'm here to help add an expiration date field for your subscribers/customers in QuickBooks Online (QBO).

If you're referring to adding the field on the invoice template, you can customize/add the field by editing the template under Custom Form Styles.

1. Go to the Gear icon.
2. Select Custom Form Styles under Your Company.
3. Select the template in the Custom form styles page.
4. Click on Edit under the Action column.
5. Click on Content and click on the header section of  the invoice template.
6. Click on Custom field under Display.
7. Add the expiration date field.
8. Click Done.

After following the steps provided above, you'll be able to add the field when creating invoices.

Let me know if you have any other questions and if you're referring to something else, I'm always here to help you. Wishing you continued success!