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Either option works if you need to back pay your employees in QuickBooks Online Payroll. For the ad-hoc pay run make sure to select the same details like the pay period ending date and the pay date, and then create and lodged an updated pay event. If you have an upcoming pay run you can add the back pay to that pay period.
You can check this article to learn more about How to create an ad hoc or off-cycle pay run.
If you need more help on the process, I'd recommend contacting our Customer Care Team through the Help menu in QuickBooks Online. That way one of our live support can guide you.
Post here anytime if you have more questions about QuickBooks Online Payroll. The Community is here to help you.