I am having a great deal of difficulty having the custom fields under "Account and Settings->Sales->Sales Form Content->Custom Fields" to show up under my Customer records. Am I missing something or is this some sort of permanent restriction?
Thanks for posting your concern here in the Community. I'm here to share some information about adding a custom field for your customer's external ID in QuickBooks Online (QBO).
The steps that you've tried will only add a custom field on any of your sales transaction but not on your customer records. If you already added the custom field, try to create an invoice and you'll see it there.
If you'd like to see the external ID of your customer on his/her records, you can edit their details and add this information. Here's how:
1. On the left pane, choose Sales then go to Customers tab. 2. Click on your customer's name. 3. On the right side, select Edit. 4. You have two options to add the external ID. You can add it to the Other box (you can view this information when you pull up the Customer Contact List report) or add it to the Notes (you can see it directly on the customer details). 5. Once done, click Save.
With regard to using third-party software, the data mapping is done on their side. I couldn't recommend what's best for this since the software isn't made by Intuit, but you can reach out to their technical support team to ask for the correct setup.
If you have other questions, feel free to comment below.