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It can be done via the command line. Open up a Command Prompt (Press the Windows Button and type CMD or navigate to All Programs -> Accessories -> Command Prompt). Then you just need to use the command below that matches your OS.
For 32 bit Windows:
cscript "C:\Program Files\Microsoft Office\Office15\OSPP.VBS" /inpkey:yourkeygoeshere
For 64 bit Windows (assuming you are using 32 bit Office):
cscript "C:\Program Files (x86)\Microsoft Office\Office15\OSPP.VBS" /inpkey:yourkeygoeshere
Obviously, you need to replace yourkeygoeshere with your product key.
Note: If you are installing a 64 bit version of Office on 64 bit Windows, the command for 32 bit Windows should be used (or just delete the (x86) part since it won't be installing to the C:\Program Files (x86)\ directory).