Hello there, hannabee.
You're right. The QuickBooks Online Simple Start version is intended to provide basic accounting functionality at an affordable price. As a result, features like custom fields are not available in this version.
If you really need to add custom fields to your transactions, you may need to consider upgrading to a higher version of QuickBooks Online that offers this feature, such as QuickBooks Online Essentials or QuickBooks Online Plus. See this link for more details: Choose an affordable plan that’s right for you.
Once you upgrade, you can use this article to learn more about the custom field feature: Add custom fields to sales forms and purchase orders in QuickBooks Online
Please don't hesitate to let me know if you have any further questions or any other concerns about QuickBooks Online Simple Start. I'm here to help.
