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#11
You do not need a key, Once the computer connects to Microsoft servers it will activate, As long as your using the the same version of Win 10 Home or Pro that was installed before even if you switch from 32 or 64 bit it will activate.
If when you are installing and it asks for a key you just press skip, I think it is still called skip it's been a while for me since I installed Win 10 and it asked.