Just did this myself, and can confirm it works well. Never used developer mode before, but found it quite straightforward. For anyone else trying to figure this out:
1. Right click the offending tick box and select Inspect.
2. That brings up a large window of code (developer tools), which will have a line highlighted. Double check that contains the voucher number you’re trying to use
3. Delete the bit that says disabled=“disabled”, and as if by magic, you can now click it!

Thanks, aahjnnot!