I understand that you wanted to add new customers to the custom field for your business using QuickBooks Online (QBO), Mushivochi. Let me help you with your concern.

In QuickBooks Online Essentials and Plus, the custom field you set in the settings will only show on the sales forms, with customer profiles excluded. In QuickBooks Online Advanced, you can add custom fields to customer profiles and other account items.

You'll want to double-click the custom field you created, verify that the correct category selected, and make the necessary changes. You can also add custom fields on the customer profiles using QuickBooks Online Advanced. Here’s how:

1. Go to the Customers tab, find and open a customer. 
2. Hit Edit, and go to the Custom Fields tab. 
3. To create a custom field, select + Add custom field. Fill out the info about the field, then click on Save. 
4. To edit an existing field, tap on Manage custom fields. This opens the Custom fields page so you can make edits. When you're done making changes, close the window to return to the profile. 
5. Hit Save to save your changes.

In addition, learn how to add custom fields to your sales forms and purchase orders in QuickBooks Online Plus and Essentials: Add custom fields to sales forms and purchase orders in QuickBooks Online.

Please let me know if you have more queries about adding a new customer to the custom field in your account. I'll be here to answer your questions.