This is working as designed, @AH48. Allow me to elaborate further.
QuickBooks Online (QBO) stores sensitive business information such as your financial records, customer information, and other critical details. For added security, the system is designed not to autofill login credentials to ensure that your data is protected at all times.
As a workaround, you can consider using a third-party application that can autofill your credentials for you. There are several reliable options available in the market that you can explore.
Additionally, you can refer to this article with the guide on personalizing your sales forms: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Please return to this thread if you have other concerns with QuickBooks. We'll be here to lend a hand.