Welcome to the QuickBooks community! I'm glad to have you join us. It's awesome to hear that you got a rebate for your sales tax. The good news is that QuickBooks Online lets you easily record that rebate. To do so, follow these steps:
Select Sales
Select the Sales Tax tab
Choose Filed from the All returns drop-down
Select Record payment from the drop-down in the ACTION column.
Select Refund
Enter the Payment date (or the date which you paid the CRA or Revenu Québec)