How do I set a productkey for office 2013 which I can then burn to disk?

What you really like to do? If you want to create an new ISO for Office 2013 with integrated P-Key, that P-Key would be used as often as that ISO is used for install Office! If it's an VL or MAK Key and you're the owner, that may would be legal if not it would be illegal.
If you would like to create already burned DVD's with integrated P-Key and that P-Key would be used only once per burned copy, that would be ok.

So you like to have the P-Key automatically added while started installation? Or what you really want?

 

This is my CONFIG.XML for Office 2007 Enterprise:

Code:

<!-- Install Office Enterprise 2007 -->
<Configuration Product="Enterprise" >
   <PIDKEY Value="XXXXXXXXXXXXXXXXXXXXXXXXX" />
   <Display
      Level="basic"
      AcceptEula="Yes"
   />
</Configuration>

When you put the key in, there's no "-". Save it where setup.exe is located.
I assume that newer editions are similar...

 

You could just use an Autostart Menu Builder for to create such scenario.There several Freeware apps available which are working with kind Macro Programming. You could also add the P-Key to display in an windows at the time the input is needed which would need the user interaction to key in the P-Key.

Also there free Installation Builder Apps available which would automate the whole process, if it's what you want?!

Question at all is, would it be all times the same P-Key and if, is that an VL or MAK Key and you're the owner?! Think about the legal consequences if you use unauthorized pirated key's and even it's against the Law if you use your own bought Single License P-Key more than once!