Hi there, ovtnvw.
We're unable to set a default account for making deposits at the moment.
I'll personally share this idea to our product engineers for future updates. You can also send a suggestion from your company by going to the Help menu, and choosing Send Feedback Online.
You can always go back to this thread if you have other questions.
In Quickbooks Online click on the gear at the top right. Click Account and Settings -> Payments, then scroll down to Chart of Accounts where you will see a drop down for Standard Deposits where you can select the correct account to record deposits.