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).
This is an old thread but anyway, this is what I get when trying that command. What is a "product SKU" ?
---Processing--------------------------
ERROR CODE: 0xC004F069
ERROR DESCRIPTION: The Software Licensing Service reported that the product SKU
is not found.
---Exiting-----------------------------