Hi there, @mariot.
I'd be glad to help you add your bank account in QuickBooks Online. Please refer to the steps below:
I've also added some screenshots for your additional reference.
I've included this article that may help you with your future task about assessing your business financials: Reconcile an account in QuickBooks Online.
Please don't hesitate to let me know in the comment section if you have other questions, I'd be happy to answer it. Take care.