Thank you for reaching out the QuickBooks Community, achontos.

Let me guide you with showing the reference number when printing the deposit slip in QuickBooks Online (QBO).

Did you enter the reference number on the Receive Payment page? If not, please follow the steps below:

1. Click the Plus (+) icon and select Receive Payment (make sure you have an open invoice).
2. Choose a customer.
3. Enter the number in the Reference no. box.
4. Select those transactions by clicking the checkbox beside each invoice.
5. Once you're done, click on Save and close.

Now, let's print the deposit slip by following these steps:

1. Click the Gear icon and choose Bank Deposit.
2. Select the payment transaction by clicking the checkbox beside the Received From column.
3. Click the Print link and choose Print deposit slip and summary.

On the Print preview page, the reference number will show under the Check No. section. I've attached some screenshots so you'll know what I'm referring to.