How can I Assign a Client reference number when adding a client. This appears not to be part of the client info fields?

Welcome to the QuickBooks Community!

I'm here to provide some details about adding custom fields to a client's profile in QuickBooks Online (QBO).

The option to add custom fields to your client's profile is currently available in the QBO Advanced version, which is exclusive to the US region. Thus, this option isn't available in your region.

However, our product engineers are working to have this QBO version to be available to all regions. For now, I encourage submitting feedback directly to them. This way, they'll add your suggestion to the list of the users who wants to have the QBO Advanced version in all regions. To submit feedback, you can go to the Gear icon and select Feedback. Then, enter your suggestion and click Submit.

You may want to view all your sales transactions in the future. You can go to the Sales menu and select All Sales. This will gives you a great view of the status of sales transactions, open invoices, and paid invoices. 

We're always here to listen if you have concerns regarding other features. You're welcome to post again or leave a reply below.