Thank you for joining the thread, Leesaonxm. 

As of the moment, adding custom fields on customer profile is not available yet in QuickBooks Online Australia.

The functionality may not be available at this point but we will forward this to our product engineers so this may be considered in the future. Also, you can send your valuable feedback by clicking on the Gear icon and select Feedback. 

Feel free to post again if you need any help. Have a nice day!