Hello there, @DannyJeffery.

I understand how important it is to add a Custom field with your forms to add some information in your preference. Let me provide some information that can help you fix this issue. 

The Intuit developer Team is the one who can help you add custom fields in your QuickBooks account in a Sandbox. As mentioned by one of my colleagues above, MaryLandT, we recommend reaching out to their forum. No worries, I'll provide you the steps in case you missed it.

Here's how:

1. Click this link: https://developer.intuit.com/.
2. Hit Contact us under Intuit column.
3. Select Ask A Question.
4. Choose your subscription.
5. Sign in to QBO and follow on-screen instructions.

Additionally, you may refer to these articles for more information about using Custome fields:

• Use custom fields in reports in QuickBooks Online Advanced
• Use custom fields for vendors and expenses in QuickBooks Online Advanced

Please don't hesitate to click the Reply button if you have additional questions about Custom fields. Have a great day!