It's awesome to see that you're taking advantage of QuickBooks Online's payroll solutions to streamline your remittances. The system automatically generates these forms on your selected schedule, so you just need to send the information to the CRA. I'll be happy to help you with this.
Selecting the time period on your PD7A forms only takes a few easy steps:
Head to Taxes and select Payroll Tax.
Click on Monthly Forms then PD7A.
Choose the appropriate period from the Select the tax period dropdown.
If you need to make changes to the available periods, you can do so by updating your remittance frequency:
Click on the Gear icon then select Payroll Settings.
Select Federal Taxes.
Click on Change or add a new frequency.
Set the desired frequency then click OK.
With this, I'm confident you'll be all set to produce your PD7A for the correct period.