Hi there, @martin24.
Thank you for reaching out to the Community. The Unique reference number it is all requirements from the bank. Your online banking is set up with a One Time Passcode.
With regards to the unique number is shown on the HSBC website. If you're unable to find it there, you can reach out to your bank, and ask for the information.
In case you've encountered an error message, please check this article on how to Fix bank upload errors in QuickBooks Online.
If you have any questions regarding banking, let me know. I'm more than happy to help. Have a great weekend!