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You can do that easily from the recovery system.
Boot to recovery partition (Reboot, holding Command-R)
Erase the partition, using Disk Utility
Reinstall macOS.
Or, if you are just returning for refund, or exchange, you can just stop at erasing the partition.
Apple, as part of a refurb, would wipe and restore the macOS, anyway.
 
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So I screwed up. Now when I turn on my MacBook Pro I get a flashing folder with a question mark.

This sucks.
 
"Flashing folder" means that there's no boot system found.
If you erased the drive with Disk Utility, then that would be the reason.
So, all you need to do is boot to the recovery system again, and reinstall macOS (if that will fix things for you :D )
 
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