Thanks for returning to this thread, avans. Let me clarify why you don't have the feature to create a custom field on a customer record.

The ability to create and use enhanced custom fields for customers is exclusive to QuickBooks Online (QBO) Advanced. Therefore, this feature is unavailable with QBO Simple Start, Essentials, or Plus versions.

As a workaround, we can add multiple email addresses for your customer in the Email field on the Customer profile using a comma.

Here's how:

1. Sign in to your QBO account.
2. Click the Sales tab and select Customers.
3. Choose your specific customer and click Edit.
4. Enter your customer's email address in the Email section, then put a comma for another email. For example, (yyyy@email.com, xxxx@email.com).
5. Once done, click Save.

I've also attached a screenshot for your reference:

Also, please note that when sending a sales transaction for this specific customer, this will sent to the multiple emails entered based on the setup.

On the other hand, you may consider upgrading your subscription to QBO Advanced for more enhanced features and functionalities.

Furthermore, you can check out this article to learn how to send statements to your customers, showing them summaries of their invoices, payments, credits, and balances: Create customer statements in QBO.

Let me know if you need any other help with creating a custom field on a customer record. Have a productive day.