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).