Hello, Jacobrose.
To clarify things, are you using QuickBooks Online Advanced? As of this moment, a custom field to track additional data of your customers is available in QuickBooks Online Advanced.
The workaround here if you're using Essential you need to upgrade your version to see this feature.
You may want to check out this article to know the differences between these plans are the availability of certain features, number of user and usage limits. Theses are the lists: Compare QuickBooks Plans.
Also, let me share with you these articles as a guide on managing customer and vendor's details in your QBO account:
Drop me a comment below if there's anything else I can help you with in QuickBooks. I'm always here to lend a hand. Have a great day!
